I'm not quite sure what's up with API...they're the longest standing testers in our hobby one would think they'd have it fixed by now. All I know is we started long ago finding out they were for sure wrong in most cases, anytime a .25 is recorded. Chiefly it was people with fully cured live rock, no source for free ammonia whatsoever, reporting .25 lasting months and months while all along delicate pods and fanworms were open and doing fine, and fish displayed no breathing issues (the opposite of all this happens with true sustained .25)
I think API is fine for high level assessments if they show 8 ppm then something is out of whack. Each kit has a shelf life not sure what the max is for salifert but I'd recommend them. I don't personally use ammonia testing at all in reef keeping its not required if you can biologically predict where it will and won't arise. Since I start reef tanks using only cured live rock, never dry substrates, I don't need to test for ammonia at any time.