I've never really looked into IPB as I was using vBulletin so far so I can only advise you what I like dislike about MyBB from a developers perspective.

Pros:
The plugin system is nice.
As a base it's secure and expandable.
Easy to manage.

Cons:
Templating - Not sure why MyBB never used a templating framework such as twig.
A lot of unnecessary code causing increased loading time.