First correction I am going to make is you need to compare JSF to Spring MVC. Spring is much bigger than just the web framework piece, as a matter of fact JSF can be used together with any of the other Spring projects including Spring Web Flow.
The 2 are very different paradigms, one is component based while the other is request based.
I would say a couple drawbacks for me are:
1. JSF abstracts away the Servlet API more than I would like.
2. If there are not existing components available to do what you need writing them is painful.
3. I think the learning curve is higher for JSF
*Disclaimer - I have not looked at JSF since JSF 2.0 came out.
These are my opinions your mileage may vary as they say.
JSF is one of the frameworks compared in link I gave in your other thread.
FYI chances are if you post this in the JSF forum you will get a totally different perspective. Since I am biased I think it will behoove you to hear from JSF users what they like or dislike about it, therefore I am going to add this to the JSF forum as well in case somebody wants to comment.