In terms of real criticisms, they basically took it from a flawed but okay mech simulation to a blantant P2W with On-Pay overpowered Clan Mechs. They made it so hard to grind that you have no choice but to pay them for new mechs, unless you want to be stuck for months with the shitty starters.
There's been some 'contention' as to how much Piranha is at fault for the P2W formula vs the other corporate entities involved.
For me it's just a barrel full of missed opportunities and misfires.
The engine and the 'feel' was always very 'meh' for me. You never got the sense of being in control of a ten-story mech raining destruction upon an urban landscape, it was more of 'window into a static and empty landscape' feel.
I want cars crunching underneath, animals and civilians scurrying for cover, small arms fire from ground troops pinging uselessly off of your hull, and lots of signs of epic conflict going on just outside of your immediate battle.
I want tall buildings being obliterated as you fire Gauss rifles and miss your target, you never got a sense that the weapons you were using were actually destructive. I want holes chewed out of rock formations and twenty story buildings crumbling.
Nope. Static window viewing a dead, uninteractive landscape.