Where the last story was both a break from the action and furthered the arc, this story is just a break from the arc to do something fun with the situation. Unless the Rhymonsters at the end will return, this story has very little bearing on the developments of this series, except maybe to make a case to Alex that the Doctor and Alex are very similar. This could be a massive drawback for this story

Except that this story is a brilliant Time War story that would only work in the setup of this series, makes good use of all characters, is genuinely creepy and manages the atmosphere and pacing to perfection.

Well played. It may not further the arc too much but it delivers on all other levels.