I'm more familiar with them for MTB, but the basics still apply to road/gravel.

You can still get flats: the slime will only fix holes up to a certain size, so if you're unlucky enough to run over just the wrong thing you'll still be on the side of the road with your tire levers. But it will seal up the smaller punctures that are more likely to occur regularly. There are products affectionately known as "bacon strips" that help plug up bigger holes to get you home, but that tire would still probably need replacing after getting back. And you can always throw a spare tube in if you need to.

Disadvantages of tubeless:

  • Can be finicky to set up with the rim taping to make sure it's air tight
  • If you ever have to replace a spoke nipple, you're probably going to have to retape and slime that wheel
  • Sealant does evaporate/dry up over time, so there is some ongoing maintenance cost that doesn't necessarily make it any cheaper than buying tubes

Slime inside a tube is a good compromise I think, doesn't have the same annoyances as tubeless while still offering protection from smaller punctures. Biggest con is just a slight weight penalty and that's not even that bad.

One of the biggest reasons tubeless is so popular in MTB is it lets you run lower pressures to let the tire conform to uneven terrain better and get increased traction, without worrying about pinching the tube against the rim (or, from the tire compressing landing jumps): it's not really about regular punctures from external objects, though it does help there. I don't see this as being a big factor in road/gravel riding. While I wholeheartedly would go tubeless for MTB (I just did my fat bike wheels), I think for road I'd go for slime in the tubes.

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