Where to even start with these two?
Mickey is a selfish, irresponsible addict. As such, she continues to defend and rationalize her destructive behavior. More damning than that; it doesn't seem like she's really sorry for who she hurts. Her concern is, and always has been, saving her own ass.
Nowhere is this more obvious than in the Season 2 finale, when she begs her ex Dustin (whom she's been sleeping with while Gus is working in Atlanta) not to tell Gus the truth. She claims it's because she should be the one to tell him, but we all know she's not going to - not until she inevitably self-sabotages the relationship next season for the millionth time.
In all fairness, I might be projecting a little because I used to date a Mickey, but Mickeys can't be "fixed" by romantic relationships. They have to fix themselves before they can function in one. Mickey refuses to be fixed (see #3).
Which leaves Gus. Gus is portrayed early on as a nice guy. Just a simple, average-looking nice guy. He gets cheated on by his long-term girlfriend at the beginning of the first season, so we start out rooting for him to make it work with the ~cool, sexy~ Mickey. But as much as I hate for Mickey to be right about anything, she's on point when she calls him condescending. Mickey is a project for him to fix - an even more problematic variation of the Manic Pixie Dream Girl.
Gus is also completely reliant on his relationship with Mickey, which would be concerning even if they'd been together for years . . . but they haven't. They've known each other for fewer than two months and didn't become exclusive until the Season 2 finale, so why is he taking all of his cues from Mickey?
They just really fucking suck. That doesn't mean they aren't deserving of love or joy, but they suck. Sorry not sorry.