I never use numpad, so that's one reason I don't care for them.
Another reason is you don't have the space between functional areas of the board, so you can't "index" them easily, can't find them by touch like with a TKL.
Yet another reason is the lack of standard placement / distance between areas, so for instance trying to hit "Home" while using the arrow cluster will end up in "Backspace" getting pressed. If you use thumb for "0" on the numpad, you'll hit right arrow.
It's just far too much of a compromise, IMHO, with the "benefit" of compact size far outweighed by the downsides.