When testing the Rampage monoblocks, 8 Pack saw significantly worse VRM temps using the monoblocks rather than just a CPU block and leaving the VRMs be. But I've not seen a similar observation for the Z series Intel boards or AM4/TR4.
You say "if you use a fan" and "if you can use active cooling" and that was my point: what if you don't or can't? Airflow over a motherboard has always been something to consider when you're watercooling, especially as VRMs have become chunkier beasts. So the monoblock idea gets around an absence of airflow by combining their cooling with the CPU. Smaller cases or confined areas around that part of the motherboard may preclude just slapping a fan over the VRMs.
Now given how chunky VRMs have become of late, it's entirely possible that the monoblock idea is no longer viable (the Rampage blocks indicate this, and I dread to think what would happen with a MSI Meg monoblock). Yes, if you can slap a fan over the VRMs then don't bother with a monoblock, but if you can't (or won't) then surely there is a tangible benefit to putting them under water, but you'll have to consider your overall loop accordingly to handle the load.