So I decided to wade into the mire that makes up the Legion forum. Some of these have been talked about here before, but these are my takeaways:
It seems that more than a few people are suffering from packaging mistakes (eg. missing components). So much so that users are complaining that FFG support has been having issues. I don't remember much packaging trouble with any IA release except for Jabba's Realm, and that was just broken pieces, not missing items. I could have a bad memory for this though, someone please correct me if I'm wrong.
Legion and IA figures are fairly compatible. The bases need modified, but the size difference can be ignored:
Every figure in Legion needs assembly. This has created more than a few issues, noticeably with the speeder bike
There's only rebels and empire in Legion (at time of writing), and there is some clamor for a third army. I really enjoy the mercenaries in IA, I just wish they would bring a third faction to Armada as well.