To my knowledge, (20+ years owning 42 classic Triumphs, overhauling and/or restoring another 20+, being on dozens of Triumph forums & online groups, and dealing with 20+ parts resellers and distributors), I have NEVER heard of cam follower issues along the lines of the horror stories of Norton's infamous "stellite tipped" problems.
As far as I know, as long as you use the proper "R" type when spec'ed, don't mid exhaust and intake, don't mix pre-oiled and oiled style, don't mix used followers with other cams or in any spot other than the one they came out of, you should be just fine.
Harris mfg (supplier to Bonneville shop I think) makes them for Triumphs and I've just sent them plans for BSA tappets and hope to order a shit load of them. There were some aftermarkets available through Norvil (no longer available) that were questionable but cant confirm any failures.
I've never heard of a single failure from any of my customers. SRM no longer offers them and the manufacturer that made them has disappeared.
I prefer the BSA A65 tappets over Triumph because the BSA tappets have a taller foot that allows for more lift. The BSAs also have a hollow 3/8" stem compared to the Trumph 5/16" dia stem.