While there are certainly areas for improvement, the P2 team does a great job in fixing bugs. They are preparing API, which in my opinion will speed up P2 adoption because you all will be able to wrap existing functionality into your UI, so there will be much more testing and much more bug reports :-).
I wanted to let you know that one, in my opinion very important, bug has been fixed:
Bug 264924 - [reconciler] No diagnosis of dropin problems.
If you add right now to your .options file following lines
org.eclipse.equinox.p2.core/debug=true org.eclipse.equinox.p2.core/reconciler=trueP2 will inform you what bundles were found in dropins/ folder, what request was generated, and what is the plan of installation. Maybe it is not detailed explanation of what actually happened, and what went wrong, but it should give you strong information about where to start - was your bundle in the plan? Was it installation problem (P2 fault) or maybe it is just not optimal to include your feature?
This is not a lot and a lot at the same time ;-).