I know they get a bum rap, but the two clamshell saws I own haven't given me any issues, nor have the clamshells my family members own given them any issues. The only clamshell air leaks I've encountered have been from the sealer between the halves failing, and I've re-used the seals with no issues. I know a 15+ year old 445 with the original seals that's been used a lot, only sorta maintained, and run with a non-sharp chain a lot and the seals haven't failed yet.
I have nothing against clamshell saws. I do however have issues with the Jonsered 450, 455, 525, 535, etc saws. They are not a very good design and certainly were not up to the same standard of the saws they replaced. I have had a Jonsered 2050 for 25 years and it has been a great saw and is my go-to for snowmobile trail work. I couldn't tell you the hours on that saw, but its a lot, and the bottom end has never been touched.