Definitely. Best place to follow a lot of the debate is LWN. See this article for starters on some of the numbers that prompt the criticism. Canonical also has developed a reputation for maintaining large patches or groups of patches for applications rather than pushing the stuff upstream. Or they fork and create a new upstream.
These are just some of the arguments I can recall popping up on LWN the last few months -- I'm sure you can find more.
Of course, the usual suspects are at work here too... jealousy for sure as the technical heavy lifting of Canonical is done by Debian, Red Hat and others who employ the kernel hackers and such. Without these folks, Ubuntu wouldn't even exist...
Anyways, not to say Ubuntu doesn't give back in their own way, but these are the meat of the technical criticisms.