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It seems like people overestimate the Sylvan faction. The new dog units are quite capable of quickly taking out archers, and in terms of initiative, demons won't be far behind, and cavalry will outpace anyone. And if the battle involves Castle, the Sylvan faction might be at a disadvantage, unless it's a cheesy Wingael strategy. I've defeated the Sylvan faction with demons in the mid-game, even when both sides had Castle. Luck often plays a role at higher levels with decent development, but instant summoning of a similar number of troops can be more decisive. However, they can still be dangerous, as their archers are quite powerful.
It's probably easier to fight against the Shadow League; they struggle a lot if you take away all or almost all of their mana. Against humans, it's manageable: they lose mana and can't effectively counter demons with summons and dark magic. You can also fight against necromancers and orcs. It's more challenging against the High Elves, and mages aren't very pleasant opponents either. It's hard to say about dwarves, but they won't be easy either, as they are too resilient. But it depends on many situations. I'm talking about battles at higher levels. Otherwise, the outcome can be different on different maps.
And the tales about the supposedly weakest demons are amusing; it seems to be the opinion of those who haven't played with them much. If you gather an army, you can easily clear the map without losses, as summoned units can simply block the approach to the main troops and absorb attacks, allowing the main forces to attack freely. This is especially easy for demons against infantry units.
It's time to understand that there is no weakest race here, just as there is no strongest.