I actually think Buffalo should have bottomed out more consecutively and longer, as it is, they've generally lacked identity and changing GMs all the time is part of that.
In 20/20 hindsight, don't trade for O'Reilly, don't try and improve rapidly from a team torn down to the studs. 'Tank' out 2015-16 and 2016-17, likely still pretty bad 2017-18. Make a lot more excess draft selections consistently throughout the five year period. Focus on adding Defensemen, not forwards in free agency. Buffalo is too unfocused. Rounds 1-3: 1 third out the door in 2015, 1 excess 3rd in 2016, 1 excess 2nd in 2017, 1 3rd out the door in 2018, 1 excess 1st in 2019 but the 2nd out the door (resulting in only a net gain of 5 draft spots), 1 third out the door in 2020.
It's like they wanted t build from the draft but stopped hoarding draft capital for a long stretch despite not being close to the postseason. They weren't focused, I'm not sure what they were trying to do at any given moment in time. A lot of their moves feel random and not calculated as part of a bigger picture.