What makes the techpriest "tanky", namely cybernetics and heavy armour is something everyone else can get as well. A Techpriest does not inherently have more TB than anyone else. For example, an Arbite can go to any arbite station, check in and requisition his equipment for a "special assignment". That means FREE carapace, assault shotgun and shock shield. Add a pistol grip to the shotgun, go two weapon fighter mixed and you have a stunning, semi-automatic fire using blaster that simply shreds things to bits.
Umm..yes he does. Machinator Array +10T. Also flesh is Weak for roughly 1 more armor per 2 ranks that stacks with worn armor. So for a given armor a tech priest wearimg it would be a lot more tankier than any other class.
Also, I didn't mean broken in the way that it makes the game unplayable and can't be challenged (that's the psyker), but that there's simple almost no mechanical reason to pick something over tech-priest for most roles.
I also gind the sniper example greatly exagerrated. Aming with an accurate rifle yelds you about 3d10+4 damage. In order to get the -10 critical effect on an average damage roll you would need your target to have less than 7 hp. Somehow I find it unlikely.