I feel like a big problem with the Capaldi Doctor is that the writers never had a clear idea of what they wanted him to be. Initially, it felt like they were just so desperate to get away from Tennant/Smith era charming, youthful, whimsical Doctor and so they went to the polar opposite. Unfortunately "crotchety" as a primary character trait isn't really as endearing as they hoped. It actually drained just about all of the fun from the character. Then when the writers tried to inject a bit of life into him, it came off as forced and gimmicky (the guitar, the sonic sunglasses, etc). Now he is in just some sort of in-between limbo where nothing about him is really well defined or endearing.
Even after this premiere, the only time I have totally bought into Capaldi as The Doctor, 100 %, was during The Return of Doctor Mysterio. I feel like its the best Capaldi has ever been and it actually stems from him doing the absolute reverse of what you'd expect from the Doctor. Usually it is the Doctor being out there and the companion playing it straight. With the absurdity of Doctor Mysterio, we kind of got the reverse, with the companions being the absurdists and the crotchety old Doctor being the straight man. This would've been an interesting route to take the character from the beginning, but I feel like that got bogged down with Clara, whose existence kind of precluded that role reversal.
I'd like to think there is an alternate universe out there where Astrid was 12's companion and we had this wild, adventurous, energetic, reckless, immortal wanna be, but never quite could be, Time Lady trying to travel through history and essentially be the Doctor and the actual Doctor being the crotchety, hesitant, companion. For once, rather than being the trickster, he would be the one trying to keep everything together. That could've actually been a really fun route for 12.
But alas, we didn't. So I feel like 12's legacy will always be that of a kind of poorly defined Doctor, who tried way too hard to be a dark and gritty antithesis of 10 and 11 and then tried way too hard to course correct when it became apparent that the original route wasn't working and as a result we got an inconsistent in-between with no real personality or defining traits. And it is a shame that that will be the legacy of Capaldi, someone who clearly adores the character and spent his life dreaming of playing him.