One of the largest weaknesses of necromancers is the lack of root spells. You will not be able to root living beings until level 34. Another weakness is the lack of good direct damage spells. Your lifetap spells and poison bolt will be your best direct damage spells throughout most of your career as a necro. You can do a little bit of everything.
Necromancers are probably the most suited class in the game for solo play.
With recent changes your pets will no longer take a share of experience in a party unless they dish out the killing blow. At this point they are not only a viable solo class but also one of the more useful classes for group play as well. NEC Top .