I'd say he's pretty decent, provided you keep his slant in mind. He's a radio journalist and not a historian, although he does have a history degree. He hasn't written books on the Mongols, but they're one of his favourite topics and he's probably better read than most on them. He definitely has a taste for the darker side, and the Mongols provided plenty of fodder for that. If you're listening to Dan Carlin, you're probably getting the "most metal" version of what's well-accepted in a decently comprehensive review of available literature, sometimes with a few of the more salacious bits from less accepted sources thrown in for fun with his warning.

Meanwhile, reading a just-released book from an actual historian may, instead, furnish you with views that are innovative and not yet broadly accepted, or just plain wrong. You really need to read a few different historians on any given topic to start to get a real handle on it. Everyone has their own strengths, weaknesses, angles and agendas. You can do far worse than starting with Carlin. He's great and making topics interesting enough to read more on.

I am not a historian though, and I'd defer to one in assessing Carlin.