I don't think that was off-topic at all. It was spot on and couldn't agree more. Tech-Priests have very specific roles and near universal characteristics shared by all of their kind. All this said, they actually fit into very few unit/regiment types. As much as a player may want to play one, a GM has to know when to say "no."
Youre missing a point. In Only War you don't play just any kind of techpriest. You play enginseers. More to that, you play enginseers on a battlefield. They cant run to the library or consult older colleague when they need a chant. They often have to invent stuff on the spot, improvise with parts usage and find unusal solutions to problems. Usually there is no techpriest around them so they wont probably suffer any consequences of their unorthodoxy. Besides theyre under constant influence of the irbsquadmates that live and die with them in trenches. That results in highly individualistic character that may drift away from its origins as far as one can imagine.
What makes you believe that? Any Regiment will have dozens if not hundreds of Enginseers working together to various degrees. They'd stick together and most likely be under the command of a senior Tech-Priest.
That being said, I don't agree with Traejun saying that Enginseers fit into very few unit/regiment types - Enginseers fit in almost universally, whether they maintain tanks or the weapons, or provide key advice on enemy technology, or simply assists in the building of whatever needs constructing.
Couldn't disagree more. Enginseers would not be part of a light infantry regiment, much less a squad. They don't need to be there to maintain lasguns. That simply makes no sense. Vehicle regiments have many... boot infantry doesn't.
Why on Terra wouldn't enginseers be present in a footslogger regiment? Infantrymen takes care of basic maintenance of their own equipment at best. Even a light infantry regiment is going to have support vehicles, command structures, maintenance issues (whether for basic amenities/functionality, vehicles, or buildings).
Nevermind battlefield advisors or strategic/tactical know-how in terms of enemy equipment and analysis.
On the individual squads level, the presence of the enginseer is no more questionable than most any other single support specialist. You'll have to make up an excuse for why they are attached to this particular squad whether it's a psyker, a priest, a bleedin' stormtrooper or an enginseer.
Thankfully, the excuse for the enginseer is pretty simple - "You need someone to go with you and analyse the forward flux capacitors of the enemy infantry and assess the vehicular capabilities when possible." Duh.
Also, you sure as hell is going to need a contingent of the Omnissiah's finest present when the camp suffers shelling, the generators break down, or the mobile armoury is run over by a squiggoth.