I personally don't see any weaknesses except general weaknesses that are associated with Linux in general. It does have some strengths that I'll enumerate quickly though.
1. It has commercial backing. Any open source project will more often than not be better off with commercial backing and in the long run. When there is monetary value to lose, there is an incentive to keep it high quality and to keep developing it.
2. It is based off Debian. This in itself has many advantages. It brings in the strength and power of Debian, but gives it slight touch for the better. It is NOT Debian though. So while it may utilize Debian tools like apt-get, it does not suffer from Debian's overly long development cycles. The software is more up to date. You are basically running a stable and supported Debian Sid distro.
3. Community, forums, user-help. Ubuntu has done a great job with it's users and community.
That is it for now out of the top of my head.