All tank companions can tank fine. It's just all about gear. Among the classes, the only differentiation between tank companions is melee and ranged. Melee tanks are like Khem, Broonmark. Ranged tanks include Kaliyo, Pierce etc. For either melee or ranged tanks, they have different skill names but if you look at the numbers and functionality, they are basically replicates. So it doesn't make much sense to say, for example, Pierce tanks well and Kaliyo sucks. There are tiny differences sometimes, like Khem does self-heal for a small amount if he does the killing blow. But that is certainly not a deciding factor whether a tank companion sucks or not. As far as I remember, the only exception is perhaps Xalek, a very unique character and which is why I love him so much. He is both melee and ranged, wears light armor, but has a higher base defense than other tank companions. Kaliyo, on the other hand, is just like other ranged tank companions. She has no special abilities like Khem and Xalek do.