I'm not an authority, but I will offer my opinion and apologize for the length of this post.
Sure, the Harley Touring Line is suitable to ride in the IBR. Many have participated, most have finished. Of those that didn't finish, some were for non-mechanical reasons. Some were for mechanical reasons. This is all anecdotal, by the way, based only on multiple readings of IBR reports, not an actual statistical analysis.
If you are the type that fits well with the HD ergos, the bike itself shouldn't cause you problems due to discomfort. My Road King had a Police seat that I had Russell rebuild that was extremely comfortable (the seat lives on with Doug Horner in Missouri). The Police seat changes the ergos a lot so that it is a little more conventional, bringing the feet underneath the rider and taking weight off the rider's tail bone. The legs forward, butt low position works for some people, and if it works for the individual, why not?
The belt drive is sometimes brought up as a weakness. Ridden on gravel it could pose a problem as a rock between the belt and pulley will ruin an IBR. If the rider is willing to limit him or herself to paved roads with only an occasional dirt or gravel road, it shouldn't be a problem. I actually like belt drive. It is easy to live with on a daily basis, it's clean, it's quiet, it's maintenance free. Belts last a long time. On some bikes, belts can be changed on the side of the road. On the Harley, swingarm removal is necessary, which is not a side of the road type of repair -- a Hotel parking lot, maybe, but not on the side of the road.
One of the things that really impressed me about my Road King was the electrical system. It is can-bus, but in my experience, it's well thought out and works very well. There are a ton of electrical accessories available and most are plug and play. I built a stand-alone harness for all of my non-HD accessories as I didn't want to cause can-bus problems. I never had an electrical issue with my bike, but have heard of others that have.
The chassis is very good and suspension can be improved. The bike is extremely stable on the highway and yet handles well for a 900 lb. machine. The weight, however, does get more and more noticeable as time goes on. Getting on and off the bike and moving it by hand and foot all get harder each subsequent day. I noticed the same thing with my previous Goldwing. It also got heavier and heavier with each day of a multi-day rally. If you can deal with the weight, and a lot of people, male and female, have, it's not a deal breaker.
Which leaves us with the engine. In my opinion, an Evo motor is probably not a problem, a properly built or maintained Twin-Cam motor is probably not a problem, liquid cooled heads on an M8 motor might not be a problem. I would not run an air-cooled M8 again. It's probably fine for the riding that 99.9% of owners will ride. What we do is a little different and, apparently, much more taxing on the motor.
To get into more detail than my report, I purchased my 2018 Road King in August 2018. It was bought by another person a few months before me who discovered that the Police setup was too tall for him. He traded it in on a lower bike at 600 miles and I came along a couple weeks later and purchased it. I performed a fly-n-ride to the Bay Area dealer that had it and rode it home down the coast that afternoon. Before putting the bike up for sale the dealership performed the 1,000 mile service.
I rode it home in temperatures ranging from the upper 90s to the lower 50s on the coast. I noticed that first day that when I would exit after an extended ride at freeway speeds the bike would idle roughly. It didn't radiate a lot of heat and didn't exhibit any other overheating-like symptoms. A little later I read about the sumping problems that the M8 motors experienced sometimes and wondered if that was what was happening. Because it idled roughly after every extended highway trip, I began checking the oil level -- sumping, or oil collecting in the crankcase instead of the oil tank will show as a low oil level on the dipstick. My engine never had a fluctuating oil level. I typically ran it at a level mid-stick and that was where it always indicated.
I tried running a few different types of oil to see if that made a difference, and it didn't change the rough idle while hot at all. By 8,000 miles it had neither improved or gotten worse. I talked to a couple knowledgeable builders who suggested the factory tune was probably the cause so I added the torque cam and tune and updated the oil pump to the latest design with the backing seal.
While the bike ran better overall, it still idled roughly when hot after an extended ride. I typically ride with the cruise control on at 80 mph -- far from over-stressing the engine.
In February 2021, at 16,000 miles, I had a day off and ran an IBR prep 1000 mile day in cold weather. The engine, now equipped with the new cam, oil pump and a good running tune, idled roughly after a freeway leg, even in 30-40 (F) degree temps. The following day I noticed piston slap from the rear cylinder during the first 30-60 seconds of running after fully cooling off. It got longer and louder in the week following that ride. Disassembly showed some scuffing of the rear piston, but no signs of partial seizure due to overheating or anything else abnormal.
Skipping ahead a bit, I began purchasing parts for a complete rebuild that would have to be completed in plenty of time for rally prep and break-in. Everything in the engine was replaced. It had new factory heads, a fresh bore in factory 114 cylinders, a new S&S crank & oil pump, new forged CP pistons, new cam, new lifters, new pushrods, new adjustable rockers. A different tuner was suggested, so I used a TTS tuner. Literally the only things reused were the case halves.
After the build the bike was dyno tested and tuned and then broken in. While the bike ran really well and had plenty of power, the same hot idle roughness was present. Subsequent dyno tunes and auto tune sessions resulted in a good running, powerful engine that idled roughly after extended highway sessions.
I started the IBR about a month and 6,000 miles later and it failed about 3,000 miles into the IBR on Day 3. While it was a hot day, it was nowhere near as hot as I had ridden prepping for, or riding to, the rally.
While its failure was due to machining of the rear cylinder .008" larger than requested, no combination of factory or aftermarket parts in either engine resulted in a smooth running engine at idle while hot.
Marcus Reynolds, who did finish the IBR this year, cut his rally off 11 hours early because his Road King and its air-cooled M8 was making so much noise he wasn't sure he'd finish. He dropped some late bonus locations and headed in to the finish hotel. Marcus had an engine failure about the same time I did in early 2021 and the dealership rebuilt it twice before he left for the IBR, the last engine was finished just days before he rode to Provo. His engine was completely stock.
Two bikes, one stock, one modified for strength, multiple engine failures.
Marcus, like me, traded his bike in right after the IBR on a non-HD bike that will be more reliable in rally and LD use.
The air-cooled M8 engine does not seem to manage engine heat well. While Suzuki had a lot of success with oil and air-cooled motors, I don't believe Harley has figured it out with the M8. The oil pump moves a lot of oil through a large oil cooler and into head galleys that use engine oil to cool the exhaust valve area of the head. In theory, it should work well. In reality, I don't think it's enough.
Any bike can mechanical out of the IBR -- JD Smith had a catastrophic engine failure in his BMW GS two years ago, so while the BMW might be the poster child for final drive failures, most engines haven't been a problem. Goldwings have dropped out due to electrical systems failures. Other bikes with assorted failures have caused DNFs. Except FJR1300s. The stats are something like 280 entries on an FJR1300 and over 900 have finished. Or something like that.
Like Donald Lowery said above, luck does have a place in an event like the IBR. The difference between a finisher and a non-finisher, or 18th place versus 48th place might come down to luck. Look at Lionel Ramos and his CVO Road Glide. He blew a compression release out of the head of his special, hand-built CVO. While it's not unheard of, it's very, very rare. The last I talked to him, he isn't riding rallies on a Harley.
I wont, either.