I've been running a hybrid 5e game with three D&D 2024 characters, two Level Up Advanced 5e characters, and one D&D 2014 character. The characters are 12th level at this point and we've been doing this since they were about 9th level. I can't much tell the difference in power between them but we all see a big jump in threat with D&D 2025 Monster Manual monsters. They're not totally evenly distributed but overall monsters seem a lot harder.While I agree that the power level of PCs has increased, I disagree with the assertion that the challenges haven't kept pace. Some subtle new changes to monster abilities make them much deadlier. For example, the life drain from a wight or wraith no longer has a save, making it much easier for those monsters to kill a PC outright. Similarly, the mummy's rotting fist turns a PC to dust if it reduces a target to 0 hp (it used to just be tied to the curse reducing them to 0 hp over time), so a mummy can kill a PC instantly and make it so they can't be brought back short of a resurrection spell (not a bad trick for a CR 3 foe). And it gets to do two punches instead of one; if I was a low- to mid-level PC and I saw one of those I'd be turning around right quick.
In my experience so far, the threat is definitely keeping up.