Yeah, it's believed to be so. Still, I totally disagree: with two boards in play, you risk to dilute too much the action over the boards so that in the end nothing is a real threat. The increased gate limit is also a big bonus. Finally, once you know a little about the game, you'll start hitting the gates fairly regularly, so that you can control everything. Kingsport on the other hand is quite brutal: the threat of rifts could become a major pain, and Kingsport Mythos will keep busy the main board, with tons of gates bursting or bouncing on seals, triggering Innsmouth. It's not like the two boards are incredibly difficult, but the interaction of the two, in the later turns of the game, can be totally deadly.
I'd say IH+DH becomes a more interesting problem with MH in. Until you add MH, shouldn't be that difficult to handle