People will level allot of criticism at the S5 shlump and the post S6 wackiness, id say this is primarily down to somewhat of an issue with the writers but predominantly it also heavily comes down to scheduling as well.
Like in general AT has the issue of basically being kinda a western Bishojo, appealing to boys and girls everywhere from 12-30. This is neat as only recently has this large appeal and area really been worked on (in the past it was a more clear cut Disney=Kids, Ralph Bakshi=mature adult humour). But it also comes with the growing pains of being relatively new territory and the writers trying to grapple with that.
Having worked in animation i can also say having a "we dont know if we are getting another season" scenario is kinda the worst. Basically you can do a major arc about some really cool villain and finish it early, but then your next season is left empty and you kind of have to start from scratch.
Or you put off your final boss and do a smaller season ender, but then you might never be able to rap up your story. Either way you are sort of screwed as you can never make a "great season" and are left with two mediocre options. This also really affected Kora and basically helped cause allot of AT's later seasonal problems+writers being weird.