After all, witches usually attack from the second half of the battlefield, which means their damage will be halved, while the Night Lords always deal full damage.

If the choice of upgrade depends on how the unit moves... I don't even know what to say... I know that invisibility takes up a turn, but if the enemy can't reach them, won't they be able to dispel the invisibility?

Erin is not the only hero of the Shadow League. I never rely on her ability. The ability of the witches themselves is sometimes annoying, but the Night Lords also have this drawback. Besides, the chance of a Berserker appearing is 33 percent. And who will cast mass spells?... Either another Shadow League hero or a mage. Or a gnome. And that's it. Not to mention that Dark spells cannot be cast.

But doesn't it matter that the Night Lords move only half the distance? Plus, witches have no retaliation when attacked.
"Full damage in most cases..." at the beginning of the battle - certainly.

I mean that the developers didn't intend for witches to be melee units, so they don't rush around the battlefield, kicking up dust like Rakshasa.
What exactly prevents the enemy? They can focus on destroying the other units, and when they're done, the invisibility will be over.

Erin. She is not the only hero of the Shadow League, but nevertheless, she is a HERO and has an ABILITY that should work.
"Whirlwind's quote" - "As far as I know, any witch can attack, even those who shouldn't be able to attack at all" - thanks, I'll check that out today.
Drawback. Night Lords don't need to engage enemy troops in melee combat, so this drawback rarely manifests itself.
Darkness. What about mass plague? Destructive Ray?
Mass spells.
Well, the Knight will probably take out all the troops first, cast Anti-Magic, and wait for the Night Lords to arrive.
The Demon will summon troops everywhere and pick off the Night Lords like the Holy Inquisition.
The Barbarian... will find a way. For example, with free Cyclops, they deal damage to multiple units at once.
The others will find a way, or they will take Chaos magic. They don't suffer much from the lack of mass spells.