The main, number one, issue would be module compatibility between those modules made for PN and IIP (So IIP would be able to run most PN modules).
Like being able to run the image gallery module, the different chat scripts, etc etc,....
Check out my newest site....
http://www.ut2003news.comYou'll see how many of the PN modules we are running.
Heavy Loads:
PN can handle heavy work loads....From Thursday last week to Sunday morning we had well over 250,000 page requests and 111,000 unique hosts....we had only one hic-cup where one of the modules couldn't handle the load and we had to disable it (AdvStats.....where every page view was creating about 150 to 160 sql requests!
) This represented nearly 10gigs of B/W. So what I'm saying is that IIP would have to be able to handle this.
Permission Systems and User Groups:
Among the best things about PN is a very involved permissions system where I can dole out different levels of authority to different Users Groups. Like I have several Site Admins in a "site admin" user group.....I have given this group a wide range of authority to do pretty much anything except those things I feel could disable the site....like not letting them have the ability to change permissions or modules etc. In another group I have news posters....so all they can really do, over and above a regular user, is have admin ability to post news without it being moderated.
These are very important things here and I just don't have the time to continue listing all the things that make PN what it is.... but these are things I would look at before I would consider changing from PN.