I do not pretend beeing a top level player, but I've tested a few time Mah AND her crew and my feelings are quite positive ....and I feel a bit sad to see so much player disliking her.
-First of all, as it has been said previously, she is a "card eater" Master, so beware ennemy models with abilities that kills you when you hand is empty by example, but outactivating the ennemy crew when Gremlin isn't that hard I think.
-Secondly she has, with the good upgrades, a really good movement: pushes;diving charge; ability to mixt a melee boost+a charge for 1AP (ok it cost a bit, but place it the good way and you may see some miracles).
-Don't forget that she works really good with the rest of her crew wich provides many pushes here and there. By this way your battle field become a real billard table....which can provides a lot of fun in your gameplay ......less in you opponent's brain