Firstly, God mode is for Sins and Shadows and no one else (2 abilities to basically make you immune to all damage and cc). The most OP aspect of UR is not that they simply get to live longer and maybe get a heal or two, but because they get free time to play objectively. By that I mean that they cannot be killed while it is up and it allows them to defend an objective long past the time they should have otherwise died. This also gives them a large advantage in 1v1, but warzones are not dueling arenas.
Let's be honest, maras bring one true piece of utility to the table and that is whatever ability they choose to use with 30 stacks built up (predation, berserk, depends on spec and circumstance really). They can't taunt or guard (pt, sin, jugg), they can't heal or use tons of CC(sorc, merc, op), they can only kill stuff and if you let them stay on you that's what they'll do. They are just like snipers, but far more vulnerable in melee range and so they need CDs to mitigate a lot of the damage they take.
I'm not saying they are perfectly balanced with every other class, but a majority of complaints come from mercs and sorcs (of which I have both, no 50 maras yet) who are the main targets of maras. When (if?) mercs get their boost to escapability (just like hybrid sorcs have atm), they will have far fewer problems with melee when played correctly.
So in blue text, you say Sins/shadow get god mode but in the red text you describe UR being well....god mode.