[Loli]

They're really fragile for the points. Guardsmen with Carapace Armour drop like flies to any of the high RoF AP4 anti-infantry guns (Heavy Bolters, Autocannons, Gauss Blasters - those kind of guns). Inexperienced/less competent Skitarii players can also end up opening themselves up with the downsides of a Doctrina Imperative - sure, they go to BS7, but WS1 (maybe 2? Kind of irrelevant either way) is a pretty significant debuff if you're in a position to take advantage of it.

Their Battle Servitors, the Grav Cannon/2-shot Plasma Cannon dudes, are really slow. They have a huge damage output, but I've seen games where they've done nothing due to being out-deployed or out-maneuvered.

This pretty much combined with flyers. The lack of Flyer and Anti Flyer in the army hurts a lot, Cult Mechanicus doesn't have any AA, Skitarii has the Onager but whenever you use it on flyers, you just feel like it could be doing more. I've only got a few games in with my Ad Mech so I'm far from a pro at this but that's my opinion. They have enough Haywire so if you can hit with Snap Shots, you can get lucky to do something but I'm becoming a of the opinion the best way to get AA in to the army is to use the data slate that's Cult Mechanicus, Skitarii and a Knight.