I just had some of this discussion the other day with a customer. He had been sold on adding sealant to his pricy sew ups. I told him to consider the down side of sealant when one can't open up the tube/casing and clean out the old and replace the valve (as can be done with a tubeless tire). We see gummed up valves, solidified sealant rumbling around inside an old set up, partial sealing results and of course blow outs with the nasty spray of sealant onto all around the tire while inflating.

But then I gave up on sew ups 15 years ago after being disappointed in the poor construction/straightness of even the better ones. I rode sew ups since 1973 prior. Still have a couple of wall hangers with them. But for bikes I ride for real it's tubes for me. I get around 1 flat for every 1000 miles. I can live with that. Andy.