Milva is totally fine as it is.
Currently, only Monsters can't match up ST's card advantage. Both NR and SK can very easily match up ST or be at the most 1 card behind.
NR can easily resurrect a played Yaevinn or any other spy in the next round.
SK have several spies of their own, the most in any faction. There's also Ocvist which is difficult to kill/banish in round 2 and 3 because of the base strength buff it gets in each round.
As others have said, holding on to Milva till the end is actually pretty risky. There are many other more common uses for her, such as getting a spy back in hand, or simply playing it at the start of a round with Roach around to get card advantage.
So, overall, it's not a problem with Milva, but comes down to getting card advantage which is one of the most important things to focus in the game.
Like I said, only Monsters have a problem here, but expecting to see some changes in the next patch to make this a bit better for them.