The biggest issue with API for Calcium, that I've seen, is inconsistency. I don't know if the kits just spoil fast, or if it's a quality control thing, or what, but I've seen a couple be somewhat close, and others so far apart it's ridiculous. Consistency is the name of the game here.
The thing I've always liked about Seachem tests is that they come with a reference solution with a known value - so you can test a sample of that to make sure your method is right, and/or the reagents accurate. If there's any accuracy issue, you can contact Seachem and they'll replace reagents - I've only really seen that happen a time or two over the years.
The only "consistent" thing I've ever seen with API is that their ammonia kit will register .25 ammonia for no apparent reason (reagent expires or something...)
Jenn