Every vehicle I drive has a flat tappet cam, chevy 3/4 ton pickup, F100 w/straight six, Bronco w/straight six, and just sold two chevy Novas which I had for years and years with engines going in and out spending a fortune at summit and jegs
Now I fool with old trucks.
I will use the break in supplement on a new motor but thats it. I don't run aggressive cam lobe profiles these days. I used to and I seriously believe
the lifespan of the cam and lifters is going to be determined very quickly by the break in. I used to prime the carb and have the throttle screwed in so it would immediately start and get up to 2500 rpm and stay there while I checked for leaks etc.
I had some pretty aggressive cams as far as lift and duration and never did buy any zinc additive.
I can't fault some of the collector car and hotrod fanatics for using it because I can not tell them for sure that it is a waste of $$ but for myself I don't worry about it anymore.