I know it allows you to defeat logia users and gives your hits more strenth, but is it the only thing it can do?
There are 3 main types of haki
Kenbunshoku: Allows prediction and to sense people hiding
Busoshoku: Armament strengthens attack and defense penetrates logia types
Haoshoku: King's haki, breaks the will of weaker people rendering them unable to function.
As for the limits of each we cannot judge that at this time. The concept of haki didn't become public knowledge until the new world came into the scene.
We do know 2 things so far. The first is from Zoro vs Pica and Tra vs Vergo. If one person's haki is stronger than another's it will allow you to penetrate it.
The second is from Luffy vs Doflamingo and that's that haki can run out and a user won't be able to use it for awhile once it does. How long and how much is still pretty unclear.