Beard trimmers are usually made for thicker hair. They offer more flexibility to trim facial hair. The blades are usually wider apart in the beard trimmers than those for body grooming.
You can totally go ahead and use that one to trim your arms and legs. You may have to be more careful while trimming your pubic region, you are bound to have small cuts using this one there, the first few times atleast. Whenever, I forget to charge mine and end up using my beard trimmer I always end up with a few nicks there :( Personally, I would recommend getting a dedicated trimmer or shaver for that purpose, a one that has a sviwel head or narrower blades.