I find it difficult to look at this episode and the season purely objectively because either showrunner’s intrusion. Moffat yet again gives imposes a lame copout at the departure of a beloved companion. He cannot allow them to be completely dead, but manages to be cruel anyway. Attempting to have his cake and eat it, Bill will be living happily ever after, sort of, with the love of her life, I guess, for eternity, probably. The price to be paid for all that is the loss of her humanity. And of course this is not the first weasely choice.
To recap the dispositions of the primary companions of the New Who era:
Rose is exiled to an alternate universe with a mortal facsimile of her Doctor.
Martha is not a doctor or married to one, and no longer a high ranking member of UNIT, but instead inexplicably running around with Mickey the idiot fleeing Sontarans.
Donna had her memories wiped.
Amy is trapped in the past.
Clara is artificially immortal and wiped from the Doctor’s memory.
I really think there is something inherently wrong with these limbo-esque dismissals of humans. Add that to the other unnecessary misdirect of this not actually being Capaldi’s final episode as the Doctor (we’ll have to wait ‘til Christmas for that), and we have an infuriating yet otherwise outstanding episode.