Stellaris' biggest advantage, imo, was that it was a game that had no expectations set by past titles. Being the first in a new series meant it was not being constantly compared to the previous games that had the advantage of multiple expansion packs. In terms of how it plays, it owes far more to 4x games like Sins of a Solar Empire and Civilization than it does to any other paradox game. The traditions system added in Utopia is even pretty much a direct lift from the Civ series - and I'm not even mad, it was a great inclusion.
For HoI4 you have the twin blows of trying to be a sequel to both HoI2 and HoI3 simultaneously, two games that got multiple expansions but took very different approaches to their game design and have overlapping but non-identical fanbases. While HoI4 is fleshed out in some areas very dramatically compared to its predecessors (i.e. industrial production), it's also simpler in other areas, and as a result you have people mad that it's being streamlined/dumbed down. The fact that it launched in a quite buggy state with balance problems and came 12 months later than initially planned just added fuel to the fire. I think people would have been overall much more forgiving of some of the game's design shortcomings if it had been out on time and didn't have so many small issues that got people off on the wrong foot right out the gate.
Despite it's issues, I think HoI4 is a game with more potential than Stellaris. Iroincally, a procedurally generated game like Stellaris with near infinite possibilities feels somehow more rigid and less interesting to replay than any other flagship Paradox series.
