It's arguable (and common opinion on the Gearbox forums) that Maya becomes less powerful as she approaches level 50 because as she gets closer to level 50, she gets closer and closer to Raid bossing and because she has the lowest DPS out of all of the characters, she's a poor choice for Raid bossing and therefore arguably less powerful than she was at lower levels because her crowd control against normal enemies and generally anything not immune to Phaselock (which is what you're normally facing pre-50) becomes irrelevant (Gee is the only Raid Boss where Phaselock has any use and even then a Commando or Gunzerker could do a better job with Singularity grenades).
Maya also suffers from problems involving damage scaling, I've not checked this as much as I should have but I think we can all agree that skills like Helios and Cloud Kill do damage that is fairly trivial at level 50 but certainly worth investing in at lower levels (we've already seen people with low level Mayas praise her in this topic).
To make matters worse for high level Mayas, she has a skill tree that is mostly focused on healing health which is often redundant as a high level character you can usually heal yourself or simply don't need health, in fact almost every character (on second thought it could be every character) has a skill that has a performance that increases as health decreases which for the more hardcore players may even make a healing Maya detrimental to the party's efforts.
Unfortunately, I could go on but if I don't stop here people may start to think that I'm arguing that Maya is a bad character which while in my opinion stands true at level 50 I can't deny that against normal enemies or non-raid bosses (I'm awfully tempted to generalise this to "Phaselockable targets" but Phaselock is not Maya's only problem) she can be one of the more effective characters despite her lack of DPS.
And res? Well I admirably have no arguments against that and even if I did I think my post is already unreadable enough.
GT: dark reaper 175 That guy who duped everyone 600 million on Fable 2.