The answer is: It depends.
In general the combo machines don't do anything well. The exception to this is a modern $300K horizontal machining center with live tooling.
For very light work the combo machines will work, but would not be my first choice. It really depends on what your end goals are. I started out with a 12 inch Craftsman lathe and an Atlas benchtop mill. If you are planning on doing actual manufacturing and production work then buying a mill and a lathe would be the best.