This isn't terrible.  It is unfortunately dampened by the fact that The Legend of Ruby Sunday largely sets up content that becomes supremely disappointed thanks to the outright awful pay-off in the following episode Empire of Death.

And that's a bit of a shame because the pay-off here is pretty cool and a lot of fun.  Sutekh's big return here is well done, even if the Susan fake-out was a bit cheesy.  The way UNIT (I like the UNIT characters here by the way) is able to go back to the day Ruby was abandoned as a baby and they still can't figure out who her mother is was really well done, extra creepy, and an excellent way to ratchet up the tension for the big reveal.  And Mel is back yet again!  She's a lot of fun here and I still was happy to see her even after she has returned a few times for the show now.

All of this is great and it really paints a picture of how empty it all fundamentally is just one episode later.  The mystery of Ruby's mom is actually enticing in The Legend of Ruby Sunday only for it to now feel kind of hollow knowing what's going to happen with it.  Endings matter, and they can affect the larger story quite a bit.  This episode has no real satisfying ending because of that - it ends on a cliffhanger here and Empire of Death is one of the worst series finales in all of Doctor Who.  It kind of sucks, even if I had fun with this episode in isolation of everything else, and I can't help but admire some of the acting and energy of the cast.