I'm sure there are tricks and that help greatly. But for an average 500-580 horse truck a suncoast kit would be fine. I've either got a suncoast or a revmax in my dd. alot of ppl local to me have revmax that last fine also. Even in 800hp trucks
The aftermarket kits do work fine in most cases. It is those weird cases or rare instances where the failure is not just to much power and a slip. Or the clutch debris has damaged other components. Or what about when the customer decided to dump brake fluid in his trans to swell the seals. Then your trans tears a lip seal after 3000 miles and you say idk I put the kit in etc.
I believe this is what MikeL is referring too. A huge amount of backyard guys throw a kit in, they may even get some help from a good builder. A huge amount also have very good results. But when you get a core on the bench that you have no idea the history of, well that is what builders deal with. I build from cores and like Mike said I have to check every bearing, and bushing, pistons for warpage and scoring. planets for cracks. Because you are shipping a trans across states to an unknown user who may be a good mechanic or not. Did he flush and clean his cooler, is the converter pushed in far enough. Do they even know how to check the fluid.
But all in all if it was your trans and it just limped, slapping a suncoast or other reputable companies trans kit in will most likely do ok. You may have some sloppy or harsh shifting you may bust a torrington because the intermediate shaft slides, but all in all it will probably work.
But then when it doesn't. the guys come on here and trouble Mike and Evan with stuck E valves and p0777 tie up and I can only imagine how hard it is for Mike to muster up the courage to answer respectfully when he knows if he did the work the thread would have never started.