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Because we have been running in circles with the performance of IS. Double weapon was a late late game issue and we have already nerf brens for it. The potential is there but munition is not something that magically appears available.
Bolster is to UKF what the T70 is for soviets in 1v1. With the difference is that you can abuse SU early on before they are able to pump out mp after mp on tech to get their light tank out.
Observe 1v1 games or if that's not for your taste 2v2. Most games skip getting weapons and the raw performance of normal bolt action rifle IS is what are triggering most people.
If you don't nerf somehow medic/pyro to be less combat effective at 5 man, there will never be a reason to not get them at all.
Now that the faction has base medics and pyro got more utility out of it, there's room to to play around not been a brain dead upgrade. Adding a weak combat wise model means that you have to opt between utility or raw performance on squads on a case by case scenario. The added model on this utility upgrades is there for survivability.
If they need any perf boost, it should arrive late game with hammer/anvil.
Royal engineers would just upgrade automatically to 5 man after bolster.
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I think putting muni to bolster would kill the faction again, more so in 1v1.