From the different orders, the Taghmata Omnissiah can field several different flavours of tech-priests. Although many are Enginseers (basically battlesmiths), there are also the Reductors (wreckers of siege and tanks) as well as the stranger Lacyraemarta (flesh craft).
The Auxillia can be configured in a number of ways to suit whichever Mechanicum preference your play style deserves.
The multitude of build is part of the strength of this unit. Perhaps it is better to consider the three different types of techno-arcana that they can choose between for evaluating their strengths and weaknesses.
Enginseers are clearly going to be useful for keeping vehicles moving. Since their servo-automata grant a bonus to this roll, their builds need to maximise on this.
Reductors are to be played as part of a more aggressive play style. Their role is one of the wrecker and tank hunter (in a very literal sense as per the rules of the same name). The ability to take a conversion beamer to help out with this is incredible!
Lacyraematra are ones to field with large units of adsecularis as they can confer a bonus to their feel no pain rolls. Indeed, I think if one is going to go down the route of adsecularis builds, these kinds of tech-priests are 100% necessary to help them out.
The tech priests themselves are relatively weak, with only a "standard" human stat line profile coupled with unremarkable upgrades beyond the power axe of the adept. They are cheap enough to compensate though.
On top of this, the servo-automata have to take pinning tests if the tech-adepts are slaughtered - they just don't know what to do with themselves otherwise! Therefore the adepts are going to be prime targets for any opposition able to pick out unique targets from an enemy unit making their stay on the battlefield precarious. It is therefore well worth upgrading one of the adepts to a Magos Auxillia just for the toughness bonus (and indeed, the Ld bonus point). Add in a second adept for backup purposes (perhaps a second unit of tech priests as well) and it should be good for a few turns at least.
I supply one build for each of the three techno-arcana below. One of the arcana must be chosen by the controlling player; they cannot remain "blanks". Although the adepts could be chosen to have different arcana, I cannot see much point in this.
Magos Auxillia (Enginseer), Adept (Enginseer), 8 servo-automata (125 points)
Purely to keep the tanks rolling. Nothing else. Don't think about combat at all. They're just here to make sure the critical Land Raider keeps rolling. Upgrade the automata with weapons if you're thinking otherwise.
Magos Auxillia (Reductor) with conversion beamer, Adept (Reductor) with conversion beamer, 8 servo-automata (4 with flamers) (195 points)
Sit back and blast away. The servo-automata here are both flesh shields and assault deterrents with their flamers.
Magos Auxillia (Lacyraematra) with graviton gun and melta bombs, Adept (Lacyraematra) with graviton gun and melta bombs, 8 servo-automata with 4 flamers and 4 multi-meltas (245 points)
In principle, this setup could work just as well for Reductors as it can for Lacyraematra. The idea here is that this mob will be near other adsecularis mobs and be using them for cover as well as granting them better feel no pain rolls. The weapons are aggressive and should be played in that manner as well.