I really hope this aspect is improved. In HOI3 all we got were "underground resistances" and partisans really did a lot more than pop up and get blown to bits which is what happens to the underground resistance militias.
Since they did a lot more than that in HOI3, I'm not sure what the problem is.
1) Partisans had a significant impact on supply. So much, in fact, that human players running the Axis powers often used collaboration government to the exclusion of all other occupation policies. Sure, some players ran with military government, but I can't think of a single good player that consistently uses total exploitation when still in the middle of a major campaign like Barbarossa. The impact on logistics from enraged partisans made pissing off the population of occupied countries a losing strategy.
2) TFH had covert ops points, and many players made good use of them to speed up or slow down advances. You can get some insane speed in advances by coupling covert ops with the right kinds of divisions (I don't abuse it like this for obvious reasons, but it works like a charm).
3) URs didn't just "pop up and die" in HOI3. URs provide accurate intel on the province in which they exist. Carpeting occupied France with URs before Overlord, even if the French Resistance never fires a shot, provides a very accurate picture of German movements and troop dispositions in the area. No need to abuse bombers to get ahistorical "bombing intel" when you have infested occupied countries with URs.
4) URs, when properly used in mass uprisings that are timed properly with major offensives, work wonders. Sure, individual UR units that spawn get defeated as the enemy reacts to them, but you can't SR through occupied territory. Furthermore, panzers stopping to fight spawned URs aren't able to react quickly to your invasion. Not only that, but they take supplies from the enemy in the province they occupy, making a mess of the enemy's supply network. There's nothing quite like arming the French and Polish countryside and spawning a mass uprising while you launch Operation Overlord or Bagration, and watching the German response get mired in URs while you break through.
Now, you can abuse UR mass uprisings to spoof the AI, but that's not what I'm talking about. I'm talking about having my forces within 5-10 provinces of the mass uprising and advancing steadily. The units on the front aren't even allowed to disengage and retreat to chase URs (remember, I'm trying to encircle and destroy them), but the units behind the line are stuck in combat with the URs and could not even plug holes in the front if the AI were so inclined.
While I'm all for making things better, I cannot understand it when people claim the previous mechanics did nothing.
EDIT: Oh, and the partisan impact on supply and URs could be dealt with by suppression units, so it's not like they were overpowered, either. You want to suppress partisan activity? There's a division for that.