So each of them has a weakness. So do almost all players. Blind isn't an especially attacking fullback but as a result of that he's more defensively solid than the vast majority the league, and meanwhile he almost serves as a bonus midfielder because of his passing, intelligence and composure. Smalling isn't the best on the ball but he makes up for it with his cool head, physical dominance, excellent judgement and timing, and leadership at the back. Herrera isn't a DM, true, but I don't see how that's a criticism of him as a player...
The 'would I be frightened if we were playing against those players' is really a measure of attackers, and none of the three players in question are attackers, so that makes very little sense.
But yeah, all you've done is picked out each player's weakness, which I can do for any player in the whole league. The point is that those three have strengths that outweigh those weaknesses to the extent that, as I've said, they could get into any team in the league. Look at City, the favourites for the title. No sensible person wouldn't swap Smalling for any of their CBs, Blind for Kolarov (or, again, most of their CBs) or Herrera for Fernandinho or even Gundogan, who has been pretty average so far this season.