I feel like one-way shields that fully block damage for one team while still allowing the other team to shoot through it are rather overpowered. This may be changing with the upcoming release of Hunter and his deployable-disabling crossbow bolt ability, but just for a moment, let's pretend that that's not happening.
I can't be the only person that thinks turtle's shield could do with a slight rework. A good sniper is a nightmare to play against, and a good sniper behind a turtle shield is practically untouchable, because you have to expose yourself to them to do anything about the turtle shield, and there's almost guaranteed to be no way of flanking it or otherwise catching it off guard, because vassili's heartbeat sensor has a longer duration than cooldown, so it's got 100% uptime and can spot you coming.
I think turtle would be a lot less frustrating to play against if the shield only fully blocked explosive abilities, and had a completely different interaction with bullets. What if the turtle shield, instead of completely stopping bullets, lowered their velocity by absorbing some of the kinetic energy from the bullets as they pass through the shield, resulting in those bullets doing reduced damage against the target on the other side of the shield. For the sake of argument, let's say the shield cuts their damage in half, but takes the half of the damage removed to its own HP pool. This way, people behind turtle shields wouldn't be completely untouchable, and would also no longer be immune to aimpunch. If this change were implemented, you'd at least have a chance at throwing a sniper's aim off by shooting at them. This change would also prevent the absolute cheese that is putting a mine in the final objective carriages on terminal, and then placing an outward-facing turtle shield inside the carriage to prevent the mine from being destroyed from a safe distance. Another instance of the shield's current function creating absolute cheese is when turtles strafe back and forth across their shield in close quarters combat, making it nearly impossible to damage them with gunfire.
The ability would still be quite useful for the team using it, because cutting your enemy's damage in half is still very powerful, especially when you have a health station set up alongside the shield. Or a bushwhacker turret. The only thing I'd buff about the shield if this change were implemented, is making players completely destroy the energy shield, opposed to destroying the metal shield generator bit at the bottom that has less health, since completely destroying the shield would no longer be necessary to get at the people behind it. Stacking multiple turtle shields might be problematic if not coded correctly though. Hopefully the turtle-stacking scenario would be affected by the law of diminishing returns; One shield cuts damage by 1/2, the second shield cuts damage by an additional 1/4, the third shield cuts damage by an additional 1/8, etc., if shield stacking is allowed to have any effect at all.