There's a few advantages.
- If you're in goat head thorn territory you can run sealant and it self seals for minor issues that would otherwise kill a tubed tire. Repeatedly. And annoyingly.
- You don't have to worry about pinch flatting the tubes.
- Related to previous point, you can generally go a lot lower in pressure.
- There are systems that can be installed to prevent bottoming out on the rim that can't be installed on tubed tires.
- It's usually lighter, sometimes by a lot.
Generally speaking most on-the-road issues with a tubeless setup are resolved with more air, more sealant, and rotating the tire, which vastly simplifies things. Finally, and probably the biggest point to me, it doesn't prevent you from running tubes in the future if you run into an issue. Often the "my tire failed to self-seal" solution is to put a tube in it until you can more closely analyze it.
The biggest downsides are related to sealant more than anything, which isn't strictly needed but is a good idea IMO:
- You need to keep an eye on your sealant and sealant levels as not only does it leak out, but it also dries out. In practice this isn't that difficult if you check your tire pressure regularly and get to know your bike over time; when there's low or no sealant, tires lose air faster.
- When you do get a puncture, it can spray sealant EVERYWHERE. I mean, it's like swinging a hose around and seeing what gets wet. Except instead of water that evaporates, you have this compound (my bicycle is latex) that sticks to things and then dries. Not a huge issue, but cleaning it off is just One Extra Thing (tm).
- It can be really freakin' messy. When I mount tires for my bicycle I do it outside, away from the house, on the opposite side of the yard, because I've seen what happens when you have a sealant explosion. It is INSANELY messy. "Mechanics hate this one trick", because it is messy. Tubeless tires were serviced in the downstairs bike servicing area where I work, not the upstairs, because of the mess.
- It won't seal large holes. Each sealant system has a recommendation. The ones with glitter can seal larger holes, but make a glittery mess instead of just a normal mess. The solution for larger holes, particularly on the tread, is to boot the tire and install a tube.
- You need a tire patch kit, not a tube kit (IMO in practice these are easier to use than patching or replacing a tube on a bike like the CT125, since most flats happen on the rear tire and we have a center stand).
- Sealing the tires properly can be a bit tricky. For MTB's there's even a term for it - 99% of the process is simple, but you want to do a really good job on the Stan's Shake (Stan's being a popular sealant brand, and the shake being part of the process to coat all surfaces on MTB tires). Generally speaking not that much of an issue on motorcycles.
- You need a tubeless valve stem, and something to remove said valve stem if you have to go back to tubes for any reason (a non-issue on our bikes since we have to do that anyway with most tubes)
- At a minor level, while not strictly required for a tubeless setup, sealant is super recommended, but it's worth noting it's somewhat hard to dispose of sealant. Sealant disposal is the same as paint, let it dry out and then throw it out. Except this is something that is specifically designed to form a skin quickly and then everything under the skin stays liquid, so it... is kind of a pain in the arse to dry out.
- Like patching tubes, plugging tires requires skill.
- Seating the bead again without a compressor is hard.
The benefits far outweigh the downsides. I run tubeless on my bicycle that I use for doing tons of miles.
