Most of the Megas are better than their Non-Mega forms. However there are a couple exceptions. Now the Megas mostly got a huge boost in stats, but some things were changed, and not for the better either.
Alakazam's best asset is his ability Magic Guard. His Mega form has incredibly OP stats, but a new ability, Trace. This is a notable downside to his normal form.
Abomasnow Is definitely better, but his speed dropped. This isn't actually that bad however as it makes him incredibly viable for HailRoom (Weather Hail + Trick Room)
Heracross is one of the MOST dangerous Pokemon in UU. This is due to his ability to run MoxieScarf. The other thing he has access to is Guts. This makes for an incredible powerhouse. His new ability is Skill Link, and while good, it's just not quite as good as his non-Mega abilities.
I'd keep listing some of the downsides, but essentially what I want to say is that the Megas got huge boosts, but a couple things have fallen, but not quite enough to make them worse by anyone's standards.