Say what you will about other Doctors era's, but most of them have introduced at least one new villain, or been the heyday of another.
1st Doctors era, introduced the Daleks and the Cybermen.
2nd, heyday of the Cybermen, introduced the Yeti and the Ice Warriors.
Third, introduced the Silurians, the Master, the Autons.
Fourth, introduced Davros.
Fifth, heyday of the Ainley Master, and made the Cybermen credible a second time.
6th, introduced Sil
7th,. only era where they didn't introduce a new villain, but at least the Master and the Daleks had great stories.
9th, Daleks were at their best since the 60's.
10th, introduced the Weeping Angels.
11th, Introduced the Silence.
12 however? Nothing. All of the new villains like the clockwork man, the Lion man, etc have been lame, but even the classic era badguys have arguably been at their worst.
The Master, leaving aside the feminist pandering gender swap, and Michelle Gomez's miscasting, Missy is still the worst version of the character.
What does she do in her two episodes except kill Osgood? And even then there's a spare Osgood so it makes no difference LOL. In her first story her plan is to give her archenemy an army of indestructable Cybermen without a failsafe in the hopes that he'll use them to destroy the Universe?
Why would she think that? It would be like if Lex Luthor had created the ultimate superweapon that could conquer the world and gave it to Superman because he hoped he would go bad?
In her second appearance meanwhile what does she do? Seriously? She bums about and is annoying for a while, then gets captured by the Daleks, then actually does nothing but save Clara and the Doctor.
Take out Osgood's death and all you'd have would be when she shoots one of her lackey's and a soldier. Say what you will about Simm but at least his Master actually did things. He conquered the earth twice, he outfoxed, the Doctor, Torchwood, UNIT all by hiding in plain sight from them, he tricked Joshua Naismith into giving him what he wanted, he freed the timelords, he tricked Martha's family into betraying her and the Doctor, he kept the Doctor and Jack as prisoners for a whole year. Missy however does.....NOTHING but kill Osgood and that's it!
The Cybermen meanwhile what do they do in their big two parter? Seriously? They pull one guy out of a plane and clomp around a graveyard and that's it. At least in Doomsday you saw them kill lots of people, destroy Torchwood, conquer the earth etc. And on to of that they are reduced to being Missy's mooks and are beaten by the power of love.
The Daleks meanwhile fare a bit better. Their first ep, Into the Dalek was brilliant, but in that second episode? What do they do? They fail to kill two people, get compared to Dodgems, hobble about a bit, and then get beaten by a clogged pipe.
Then finally there is Rassilon. Once the mightiest and most feared of time lords who gets beaten by the Doctor in like two minutes when the Doctor just looks at him.
The lack of strong villains in Capaldi's time was probably one of the biggest problems with it TBH.