On the topic of the game itself, I'm on the fence about getting it. How does it compare to HoI 3?
Well while it's important to realize they're different games, HOI IV is no HOI III. The quality is just a bit lower. It's full of good ideas though, and given DLCs and modding it has a lot of potential to be a great game. As of right now though? Barebones. I would wait.
And can AI nations (particularly the US) actually succeed at naval invasions this time around?
The AI is much better about Naval Invasions, I've observed a successful solo invasion of Italy, as well as the Allies using present beachheads to transfer forces and reinforce their position in Europe. Having said that, some of their success comes from the AI's tendency to not defend coastlines heavily. ESPECIALLY FRANCE'S. The AI overall however, is a bit looney, and doesn't really consider the war past short-term war goals. For example, they will prioritize taking the big VP prizes without really taking into account the precariousness of their situation. That translates to GB desperately invading Italy to capture Rome in 1941, which doesn't really make sense as it tied up much of their forces with no real positive benefit.
EDIT: Specifically, the well-know Black Ice team is beginning their HOI IV overhaul which is sure to hold it higher standards, wait for that.