The ending was second-most disappointing bit for me, honestly. Missy's reintroduction was the most.
Handmines? Scary. Cheesy, but scary.
Child Davros: dropped a WTF.
Missy's back? First off, didn't the Brigadier take care of her so the Doc or Clara wouldn't have to? I'd have thought an overcharged Cyberman gun enhanced by Time Lord tech should have done the trick - or, if not, would have at least triggered a regeneration!
Then, of course, there's the "no body" question. And, thinking about it in hindsight, Missy did "die" then, under very similar circumstances to the way she "died" now. Clearly, that vambrace time-machine of hers teleported her away and made it look good to those doing the "killing." This clearly also happened with Clara and the TARDIS.
We've seen what happens when the TARDIS dies. It creates a vortex of time and blows up all of space and time, getting rid of stars and such, and replaces the sun of the local system with itself, blowing up perpetually. That is not what happened here.
So, presumably, a TARDIS has a similar "get out of destruction" system in place.
No, where I'm disappointed? We've already seen that the TARDIS will admit the Impossible Girl with nothing more than a finger-snap. I was fully expecting her to fake-out the Daleks, dodge towards the TARDIS, snap her fingers, bolt in through the door and shut it behind her, then leg it to the console and throw the old girl into gear as a dozen Daleks and their ceiling turret did their level best to exterminate it. Could have gone full Star Trek with that, sparks flying, instrumentation exploding, ceiling girders falling, etc.
But the worst part was Missy's "reintroduction." Frankly, the idea that an entire team of snipers - whose whole JOB is to put the target down if they do something untowod - would hold fire while not just one but TWO of their fellows are just disintegrated - is absurd. Why even bring them - why bring ANYONE!?
Frankly, Clara should have said "You know something, Missy? I have three words for you: All units, fire." Missy then does her blue "disintegration" trick under a hail of projectiles, causing Clara to say "well, that at least clears up how she did it. I know bullets aren't supposed to do that, and I've seen a Time Lord regeneration, and that was not it. Now, where's that time machine we should have in our own archives, eh? I need to go find my friend."
The Doctor, rocking out with an electric guitar, riding a tank, however?
Awesome. Entirely nonsensical, of course, but when you're a Time Lord and you're convinced that it's the end of the line, you may as well go out rockin'.