Mirajane isn't underrated.. She's overrated.. She's fought 3 battles(no I don't count Jenny)
She beat Freed and lost against Azuma and Seira because they were more powerful than her.. Just because they were rivals before Lisanna's death doesn't mean they're on equal footing now..
Feat wise... Erza wins mid difficultly
She's always had better feats and has more experience
So no she's not on Erza's level let alone stronger
Sure, I agree that Erza is stronger, but she's not getting mid-diffed, mid-high will probably be the best conclusion.
She only lost to Azuma because Lisanna was on the brink of extinction and she couldn't concentrate on her fight. There are a lot of factors most people don't take into consideration when basing Mirajane's power on that fight, these are the following:
- Mirajane still didn't have her complete power back then.
- Mirajane was sort of exhausted after fighting Elfman and Evergreen, who she lost to due to some unfair trick that the duo pulled.
- Mirajane had her sister tied to a time bomb.
- Mirajane couldn't concentrate as most of her focus was on Lisanna.
- Mirajane saved Lisanna to the time bomb by giving her the remaining magical power she has so she would tank the time bomb. Mirajane on the other hand took the full brunt of the bomb without a shred of MP in her body. Of course she would lose.
If the adverse conditions had not been on Mirajane's side, and it was a fair fight, it was obvious where the fight was going.
She was faster, her nukes were better, her hand to hand was better and so are a lot of other things.
As for Sayla, she lost because she's strong. Even Erza could've been defeated by that woman, or maybe it would be a tie between Sayla and Erza.