I think one of the “sleeper” units from the new Adeptus Mechanicus codex (and perhaps across the 40K meta in general), is the Sydonian Dragoon squadron using the rules from the “Stygies VIII” Forgeworld.
The Stygies VIII dogma gives all enemy units a -1 to Hit when shooting at its units from greater than 12” away (so, basically identical to Raven Guard and Alpha Legion tactics, except that it applies to ALL Stygies VIII model types, not just infantry/bikes/dreadnoughts). This is already an outstanding dogma, especially given the heavy shooting focus of 8th edition right now, but it is even better when combined with Sydonian Dragoons and some of the Stratagems available with them.
Specifically, all Sydonian Dragoons get the “Incense Cloud” special rule, which grants them a natural -1 to Hit for all shooting attacks against them… combined with Stygies VIII dogma and now all units shooting at them from outside 12” have a -2 to Hit (so Space Marines and their equivalents will hit on 5+s, elite characters will hit on 4+s, and Orks cannot hit the Sydonian Dragoons at all!). While the Dragoons are naturally only T6 with 6W and a 4+ save (so, pretty light-weight as far as vehicles go), you can use the Shroudpsalm Canticle to give them “free” cover (upping their save to a 3+), which combines with the -2 to Hit and their built-in 6++ save to make them VERY difficult to kill. For example, on average it would take the following to kill a SINGLE Sydonian Dragoon from the Stygies VIII faction:
-9 x BS3+ Lascannon shots
-16 x BS3+ Overcharged Plasma shots
-108 x BS3+ Heavy Bolter shots
As you can see, with all these layered buffs a unit of Sydonian Dragoons are quite survivable against all kinds of shooting (their -2 to Hit for shooting attacks against them also makes Overcharging Plasma shot when firing at them VERY dangerous, with overheats occurring on a 1,2, or 3 for most units). In terms of mobility, each Dragoon has a 10” movement as part of its base profile (which never degrades, no matter how many wounds it has lost), making it one of the fastest Adeptus Mechanicus units in the codex. On top of this, for 1CP you can use the special Stygies VIII “Clandestine Infiltration” stratagem to set up a squadron of Dragoons anywhere on the battlefield that is at least 9” away from the enemy, putting them right up in your opponent’s face immediately (and keeping them completely safe from hostile firepower until they arrive)… importantly, this occurs after all deployment/roll to seize is complete, but before the first player turn, meaning that the Dragoons still get to move when your turn come.
This means that, if you have first turn, or if the Dragoons survive enemy shooting for one turn, they can Infiltrate 9” away, then move up to 10”, guaranteeing that they are 1” away from an enemy unit and that they will make their Charge roll! Alternately, you can use the special Adeptus Mechanicus “Dunestriders” stratagem that allows Ironstriders/Dragoons to advance 2D6” (vice the normal d6”) for an average of 17” movement (and max potential 22”) per turn to get them into position to grab an objective or make a charge in subsequent battle rounds.
Speaking of making the charge, with their superior mobility and survivability, Sydonian Dragoons have the ability to hit VERY hard with the help of another new Stratagem, “Conqueror Doctrina Imperative.” This Stratagem affects any selected Skitarii unit and, for 1 CP, gives them +1 to hit with their close combat attacks for a turn (or +2 to Hit for units with Broad Spectrum Data Tether or Enhanced Data Tether, which applies to Sydonian Dragoons). This not only gives Dragoons a 2+ to Hit with all their attacks for a turn, but makes it so that, instead of getting three hits for every roll of 6+, they get three Hits for every roll of 4+ with their Taser Lances. So, what does this look like practically on the tabletop? A unit of 6 Sydonian Dragoons will generate 18 attacks on the charge… with “Conqueror Doctrina Imperative” activated, that means they hit with 15 of these, plus another 18 hits from the “exploding” Taser attacks on 4+s, giving you an average total of 33 x S8 AP- D2 hits on the charge. This will translate to the following average damage against the targets listed below:
-Guardsman Equivalents – 16-18 dead models
-Marine Equivalents s – 8-9 dead models
-Terminator Equivalents – 4-5 dead models
-Rhino Equivalents – 14-16 wounds (1 dead Rhino)
-Land Raider Equivalents – 4-6 wounds
-Wraithknights – 10-12 wounds
-Roboute Guilliman – 6-8 wounds
As you can see, this is very substantial output for a single unit of Dragoons on the charge, allowing them to effectively take on a whole spectrum of potential targets efficiently (you can also Infiltrate in a Stygies VIII Techpriest or Dominus with the Omniscient Mask Relic nearby to make the Sydonian Dragoons even more accurate on the charge/generate even more hits, if you really want to amp up the damage even further).
Finally, with all of these abilities in the mobility, survivability, and combat power departments, the Sydonian Dragoons are remarkably affordable, costing only a Fast Attack slot in one of your detachments and just over 400 points for a full units of six of them. Even outside of close combat they can be very effective at grabbing objectives, screening the rest of you army from enemy assault units, and/or playing a role in the psychological part of the game by threatening critical parts of his army with a fast-moving, hard-hitting assault unit. Whether you are playing a pure Adeptus Mechanicus army or an “Imperial soup” style force, it will be hard to go wrong with a unit or two of these mobile, hard-hitting walkers in your task force.