Overall I enjoyed the episode, Sacha was great as the Master (I’m gonna hold out on the explanation about where he sits in the timeline till I see the regeneration. Persnickety but I prefer to see it through) but I agree with what’s been said before re: Dr Ruth and Captain Jack. At the moment I’m unsure about the retcon. It changes everything about the Doc and is a massive risk to take.
There’s also now a plot hole as if Ruth came before Hartnell, then there’s no reason that I can think of as to why she’d have the police box TARDIS when the chameleon circuit broke when Hartnell’s Doctor landed in 1963 London in An Unearthly Child.