I don't see how you can say the tactics were to blame.
He is certainly responsible for drastically limiting our options tactically by leaving us short of wide players, which was fully within his control when he decided to sell two wide players (Snodgrass & Pilkington), sell another player who can do a job out wide (Surman), and loan out two other wide players (Bennett and Jacob Murphy), whilst signing no wide players apart from Lafferty, who fans almost universally seem to be believe should in fact be played through the middle, where he isn't going to get any games due to not being as good as Hooper or Jerome.
His biggest failure in my honest opinion has been player recruitment. Some bizarre decision making there, particularly the decision to sign three reserve centre backs who he didn't ever want to play (and not realising that the best of the three was half decent until almost half way through the season).
I'm also really disappointed that the man who led our kids to the FA Youth Cup success hasn't given a few opportunities to the various talented kids that we have. What has happened to Jamar Loza? When Grabban first hit that run of really bad form, when Hooper wasn't ready for 90 minutes, I was wondering why we couldn't have seen 15 to 20 minutes from him. He was good enough to give Arsenal a fright!
BUT, I like the guy, his resignation is damage limitation, and I'm glad that he has taken it upon himself to avoid being remembered as a Peter Grant and has learnt from Bryan Gunn's mistake. We'll benefit from keeping Adams at the club, hopefully in charge of the youth system, and he'll benefit from remaining employable. This is the best outcome for all concerned.