Auto-translated
Of course. For example, I think that Necropolis and Haven (unless you drag it out) have problems against Demons.
If it weren't for the Staff of Sar-Izzy, magical races would simply fall apart against Marbas. But this is on generated maps (where you can level up without delay), i.e., on maps with chests.
Some people practice playing with a low monster strength to make it faster. Otherwise, it gets annoying when a Necromancer runs around with several Furies, escaping a crowd of Golems, or when Demons summon units every turn.
Marbas is generally one of the best heroes for Demons (if it weren't for the Staff of Sar-Izzy again). He has very strong resistance. (against magical races) In general, at level 20 with magic resistance, it will be: 5+20+15 (and possibly +10 from boots or even better +20 from the armor of the Forgotten Hero), totaling about 50% resistance (boots are not difficult to find), and with the armor, it's 70%, which is not bad at all.
On maps where it's impossible to get a stronghold, Nibros is decent (because with tactics and a ring for initiative, you can get horses and dogs on the first turn, and the fire dogs with luck will do significant damage).
But this is riskier. You can, of course, take Nimue and summon units quickly – more units than you have yourself (by the way, this is a serious tactic). Sometimes brute force solves the problem.
But Deleb is only good as a hero with machines at the beginning, and that's it.
Recently, we played Demon vs. Demon. Deleb against Nimue – Nimue killed him with brute force. Deleb took machines to level up, but then at level 24, he reset his skills with the mentor in favor of Darkness and Surprise Attack (a very cunning tactic).
By the way, I often take attack speed, but I've noticed that I rarely get to use it because Darkness/Teleport turns out to be more important (although attack speed is very useful for troops).
Imps are not that weak. They are just a little more difficult to play than others. Of course, if you play against cheaters, then I apologize, but if you play against average heroes, it's fine