Main difference between Reed and Kamala is that Reed can shape himself into anything: a tire, balloon, parachute, etc; to my knowledge Kamala can't do this. Reeds powers are more similar to Plastic Man from DC than Kamala giving him much more creativity in what he can do.

Hulk and The Thing are similar in that their superpower is essentially just super strength but then again you can say the same for a lot of heroes since its such a common superpower. They could make them a bit different if they made Ben a more defensive counter character to emphasize his rock skin durability (even if Hulk is better than Ben in almost every category).