Diamond testers

Hi I am looking to buy a diamond mossianite combo tester. I try to read up on presidium multi tester 3 and gemOro ultratester 3.
I can not find to much to read about them
More then some have problems with inconsistency when testing with known stones. Is this mainly a user problem or can the testers be trusted for a quick check on small stones where it’s difficult to see if a stone is double refractive or not.