In terms of the OP, I haven't reviewed the DLC yet (I like to have 20-30 hours with a strategy-type game before reviewing), but I were to review it now, it'd definitely get the thumbs-up. Sure, there are rough edges (which I've posted about plenty enough), but I really like the fundamentals of the new supply system, and outside of sparsely populated areas (ie, with fewer supply hubs) as best I can tell it works really well. I just finished a Spanish Civil War, and it made the decision-making and strategic considerations far more interesting than pre-NSB/Barbarossa. As a ship person, I couldn't possibly say anything other than Man the Guns was the best expansion, but even then, I think the new supply system makes NSB a strong contender.
I firmly believe that the supply system in HoI4, with some nips and tucks, and some improvements to how information is surfaced to players, will be seen as one of the "great innovations" of HoI4 in years to come, and be the basis of supply in HoI5. And I say that as someone who has not been shy posting about its shortcomings (in no small part because I want it to be successful) - but it's important to see the big picture, and there is a lot to like about it.
Focus-tree wise, I've found the Stalin paranoia system works well. Regardless of one's beliefs of Stalin, there needs to be some kind of mechanic in place so that forces players to purge, or the Soviet Union will be significantly more powerful than it was (probably, all else being equal - noting that it's obviously a guess, it's not something we can test) and the balance of the game is out of whack. So for those not liking the purge system, what would they prefer? There has to be something, and I can't think of a better contender than paranoia (it surely wasn't rational optimisation!)
As always with focus trees, sure there's a lot of somewhat improbable but fun alt-history content which some people may not be keen on, but the game is well set-up to support historical play (I'll be playing historical for a while yet, before I switch over and try the alt-history stuff - and I know there are some rough edges here as well, but in the context of the genre and the sheer complexity of the game, these are pretty understandable) and there's nothing forcing players to play the alt-history stuff.
The new doctrine system also adds more depth - it's still early days with me in it yet, but I think it's both more fun in vanilla, and broadens the scope for moddability as well.
So given all of that, it's not hard to see at all why it'd be reviewing well. It is complex, and the rough edges may stand out more in certain situations and for certain playthroughs, so I can see why some people might be disappointed initially, but I think it's got a heap of potential. For anyone finding it a bit hard-going now, I'd strongly recommend sticking with it, or at least taking a break for no more than a week or two and coming back after the next patch to see how it's going.