4/5 stars. A very reasonable entry in the elites section.
Tech priests come as a covenant within the feudal ranks of the forge worlds. They are the lesser magi and magos of the worlds and are frequently enginseers, although others also exist.
Strengths.
The best thing here is the Battlesmith rule all round. The ability to upgrade to a Magos Auxillia is a good thing as it then provides access to artificia cybernetica where required. The servo automata can also be upgraded to better suit the unit's needs.
Weaknesses.
The BS=3 can be upgraded with the Magos, but the baseline of this is a problem. The servo automata are interesting with T=1, but only a 5+ save makes them risky to take multi meltas on each of them.
This fundamentally is a unit that you need to think about what you're doing with it. Pity that the magos or priest cannot be taken as an individual cheap enginseer really. Can't have everything I suppose.
Builds.
Tech-Priest Auxillia, 2 additional tech priests, 1 magos with cyber familiar, automata with servo arms (160 points)
A general purpose enginseer squad. Choose Reductor here and add on a graviton imploder? The beamer is interesting, but tough to make work in the elites slot as a backline unit?
Tech-Priest Auxillia, 2 additional tech priests, 3 graviton guns, 4 multi-meltas (250 points).
Very, very expensive. Probably plenty of better approaches. But take Lachrimallus to stay alive a bit longer to cause mayhem.