Uninvited guests.

By the way, if I build a Blazing Cavern, will the Witches' Sabbath destroy it? Or does it only target the Capitol and the Castle?

Only monetary and defensive structures

Added 31 minutes ago

Also, all heroes who, thanks to their specialization, cast a spell at the beginning of the battle, give 1 turn to the enemy hero, as their ability is considered a turn.

not 1, but approximately 0.5 relative to the enemy hero (after all, he doesn't go immediately). Moreover, a Warlock, Wizard, or Necromancer can minimize this gap with the help of skillful spellcasting, for example. A Knight will only fall back by 50%, i.e., approximately to position 0.5 on the ATB (if I'm not mistaken), if he has the "Righteous Fury" ability, which can be obtained during the first level-up. This concerns the disadvantage of specializations.

Regarding the advantages:
- High Elf, in addition to dealing damage to enemy stacks at the start of the battle, stuns them (if you don't unlearn the Air Elemental Lord from the mentor), which almost guarantees that the main stacks will go faster than the enemies at the beginning.
- A huge initiative advantage is gained by the Knight when casting mass haste for 40-80%. Wizards with mini-artifacts for initiative are left in the dust.
- Warlock, in addition to setting enemy archers on fire, also removes 50% of their defense and that of large creatures (if the Fire Elemental Lord is learned), which Furies and Riders will gladly take advantage of.
- Wizard can get an extra stack that is guaranteed to go first with a developed summoning skill. The hero himself also moves faster.
- Orc can counter even Wingard with his ability.
- For the Necromancer, a usually useless stack can become one of the leading ones.
- If the Demon slows down all enemy creatures at the start, casts Berserk or Dominate, the enemy will definitely be in trouble.
- And the already tough Gnome will become almost invulnerable to ranged attacks (especially good when defending the Castle).

So, IMHO, the advantages far outweigh the disadvantages, and the initial ATB setback is rather a small limiter that prevents the heroes from becoming super overpowered.

However, the fact that heroes cannot be returned to the tavern is bad news. Perhaps this cannot be overcome.