The demons main advantage is that they're pretty much endless,Hell keeps popping out more demons and will never run out of them.

The flood has the clear advantage on the way it spreads,while the necromorphs are far more unpredictable giving the amount of different forms they spawn.

Demons and Flood are a bigger threat but Isaac had a far bigger disadvantage compared to the other two,all without taking the hallucinations in account.

The main advantage of all 3 is that they gain new soldiers from the bodies of their enemies however since the demons use souls for every body the necromorphs or flood gain the demons have a chance of also gaining a soldier. On top of that demons can't be converted cuz when they die they turn to ash. Demons also tend to use a lot of fire attacks which the flood have a confirmed weakness too and I imagine necromorph flesh dies when cooked.

The demons have the fastest terraforming process out of all three as well and the most fortified bases since their attacks come from hell outwards instead of a fixed location. So effectively anywhere and everywhere can be a demons terf and the process of making it so is rather quick, and their home base is almost nigh impenetrable and highly mobile.

The demons overwhelming power is also what makes Doomguy IMO the most powerful FPS protagonist ever. Since he's the only man to have stopped them multiple times.