One thing I have seen on Very hard but haven't so much on lower levels, is the way the AI invades via ships. On VH it will typically just land a general and a unit or 2, then buy out every single merc in the area and attack with that. On lower levels where it wasn't able to do this because of funding it would actually load up a half stack or so with the general.

It does make things a bit more interesting when the AI can land a general and bam instant army.