Why do people say MeGardevoir's ability is bad? You can have Gardy enter the field, make use of whatever ability you wanted and then Mega Evolve. Can't you?
The problem becomes that Gardevoir with Trace is just so much more valuable. As is, if true, Gardevoir would enter the field and Trace, say, Water Absorb and get a free switch in on Mantine (sake of argument). If Gardevoir Megas the next turn, she loses Water Absorb for Pixilate. It just isn't useful when you could have a setup like that.
If life gives you lemons, sell 'em to Tom Nook. Black 2 FC: 0132-9980-0476 (Aki), 3DS FC: 5284-2520-2972