I think they are less reliable than mild rollers these days. Risk of not getting broken in properly and then having to use special oil for its entire life

Too much risk with a flat tappet. And a mild Comp hydraulic roller will pretty much live forever. The factory gives you everything you need to get it done reliably? Why even bother going backwards with it?

Those hrs are for blue motors which use high lift vs duration cams. Merc also used lower psi valve springs (than most people would use for these) for some reason.

Way more to it than just flat tappet VS roller regarding top end intervals.
Anyone with rusty mid 90 Chevy trucks running around your area?
They're everywhere, still running with roller cams after a gazillion hours and no top end service.

Yes I am quite familiar with roller cams and no I am not using a little stick like what is in an old l19 7.4.

This deal is right around where the 561 cam was, 22X/23X and mid 550 lift. A roller would certainly work but a good flat tappet setup here will last longer. And by good I'm not talking about an EPC cam you are buying from the 20 year old "tech" reading out of the Comp catalog :p