Part of the problem is very unhealthy mechanic of burn - damage over time on pyro. Every raider or mex or windmill it touches just dies.
I suggested long time ago to rework all fire to make area damage over time instead a unit status - more kodachi style. This way only sluggish units or statics would suffer from fire.
I think that mostly Pyro is the problem, not only for shields. It's a riot raider and its main problem are static defenses. I think that it could be more reliant on its jump ability and less generalist with changes:
5% speed reduction and 5-10% hp increase for compensation. HP increase is optional, as combined with jump ability it would make it better against statics. Yes I'm aware that speed reduction is non-linear, while hp above certain treshold is more linear.
Factories tools to "counter" Pyros:
Cloakies: Tick(?), Zeus, Scythe - zeus super slow
Shields: Felon, shielded Outlaw + Bandit (?) - super slow
Spiders: Venom, Redback, sufficient amount of Fleas - redback very slow
Hover: Mace, Scalpel, maaybe enough of daggers
Jumpers: Puppy (?), Moderator, Jack
Rovers: Slow dart + Scorcher, enough of Fencers, Leveler
Tanks: Panther, Banisher, Welder (?) - prenerf version for sure
Gunships: Harpy
Planes: Raven
In this picture, Shields look the worst as they are the slowest and most expensive counters. Your economy most likely will get raided.
Commander with lightning gun also helps a lot.
@edit:
Speaking of racketeer, I think in comparison to its previous state it's more healthy to the game and less reliable for the user. Maybe decreasing its cost by 5% could be discussed?