Boris Turret is in a Good spot, but he needs a better passive perk
With the new changes to His turret in S5, It's now able to deal significant damage to enemies and can finish infantry or vehicles off.
So he is in a good spot but he still needs work with a passive perk.
Sentry Operator is a perk that relies too heavily to his gadget, and compared to the other engineer specialists, its also very situational, where as Critical repair is useful for vehicle players or as a mechanic for the team, and Armor Hunter is useful to spot vehicles for the team to take down.
With the other classes, even they have some use like Dozer's blast resistance allows him to survive a SPH Stick.
Boris' current perk honestly should have been part of the sentry while he had a different passive perk. It could be being able to carry and place more mines to a total of 6, or carry/ spawn with more rockets, or repair nearby vehicles while inside one to list a few from past titles.