I am big fan of Portal Technology and i feel that it is next big thing.
These are few things which you can not do in portlet
1) You can not generate arbitary markup.(Portlet which generates image)
2) Portlets are not url addressable. You can not send URL of your Portlet application to someone.
3) I think Portlet development is little bit more restrictive then Servlets.
But your right you can not say that if a technology is good that means it will solve all possible problems.