ok then
point 1.) so what? we can also bring immoral units bastiladons. doesn't mean they are any good and morathy costs more then 2 of them let alone how much both of her will cost.
No, a bastiladon is not immortal. Morathi, with her max 3 damage/turn eventually reaches a state where she is immortal for the remainder of the game. E.g. when there's only 2 turns left and she has 7 wounds she can do as she pleases without worrying about any consequences because it's literally impossible to do 7 damage to her in that case. A bastiladon on the other hand is always killeable, even if you only have 1 turn to do it in.
As for contesting objectives, she's more than capable of beating a MSU of say Skinks to a pulp. It might take a turn or two, but the skinks won't be able to fight her off. And with the shared health-pool and max 3 damage/turn she won't really care about any damage she suffers in retaliation because her big monster form is probably off fighting something and getting stabbed 3 times per turn most of the time anyway, making most damage she suffers irrelevant as that'd just be absorbed by her iron heart. Providing an immense amount of freedom to her caster form compared to any other caster. She'l be much less concerned about screens because of this.
This is also super annoying to deal with as her opponent as you very quickly run into the situation where an entire unit's combat potential is wasted. You only need to do 3 damage on one of her two forms before you reach the cap for that turn. If she enters combat with both forms her opponent is basicly guaranteed to have one of his units do 0 damage even if he'd roll perfectly.
beyond that she has a good CA that doesn't stack with the other CA that let you attack again and a bad spell don't know why her being unkillable is a problem honestly i would just ignore her.
Cuz it's not the least bit fun to deal with. Especially as ignoring her only halfway works, given that she has both long range spells & a fairly decent melee profile split over her two models (decent enough at least to punish you if you don't take care of her). She might not be the best, but she's going to have an impact even if you try to slow her down with screens. And those screens aren't going to last long. Not to mention those screens would probably be more usefull blocking the charging doomstar of witch aelves.
On top of that, even if your screens slow her down enough and aren't needed for other things, if you don't do enough damage to her you eventually reach a point where she literally becomes unkillable no matter what you've got left on the table, which on occasion is going to win her games. Not because you lack the firepower to take her down, but simply because you can't do more than those 3 wounds worth of damage. There's going to be games where Morathi wins the game by say facing a horde of buffed up plaguemonks on her own and only surviving because she happened to have 4 wounds left at the start of this turn, never mind that the plaguemonks did 30+ damage or something equally absurd, the iron heart absorded it all.
anyway tl;dr: 2 models with a shared health pool, a broad range of powers (magic, melee combat, fairly fast) & a rule like iron heart is just an annoying and frustrating thing to face. Even if it's not necesarly the most powerfull thing.
On the bright side, I like the idea of the shared health pool. That has potential.