My Myrrh...

Not bad at all IMO.

you didn´t specify which myrrh you got :) but either way both of them really want their defense as high as possible, because their weapon scales with it. so i´d prefer a -atk +def one even over a +atk -spd one (-spd +def is optimal btw.^^)

Either Myrrh with +Def is awesome regardless of her bane. I have normal Myrrh with +Def -Atk and she's one of my best units Build her up and she'll prove her worth..

Don't focus on the -atk; look at how hard she actually hits and if it's not enough to get through reds with res in the mid 20s to maybe low 30s then I think you can say it's bad. Similar with other units - if they can still win same-colour matchups when their opponents don't have like 35 or 40+ in the target defensive stat, I'd say -atk isn't really a problem.

And if you're dealing with a flier, don't forget that goad fliers will cover a lot. Even hone cavalry can cover atk buffs on some horses.