every faction has serviceable HMGs and AT guns (dont say the 50cal is shit, its not. dont say the USF at gun is shit, its cheaper than a pak and also has great veterancy). you can argue that the leig/pack howitzer are different than mortars, but theyre still most definitely indirect fire. the brits definitely get the short end of the stick with the mortar pit, but its still 2 mortars no matter how you look at it.
the only thing that some factions DONT have are snipers (even then there are pseudo snipers like pathfinders anad JLI) and UKF dont have rocket artillery or flamers.
the .50cal is a good weapon but the crew has cost/durability issues. the 57mm is fine against mediums but has scaling issues against anything bigger than a IV, which is a problem because the only effective AT weapon against Vs/heavies that USF has costs quite a bit of fuel (there isn't a balance issue with the jackson, it just makes it harder to get out than an AT gun) and can only fight vehicles by kiting. OKW doesn't have a non-doctrinal MG and even when it suppressed the kubel was a less effective suppression platform than a setup team after the first couple minutes. brits have the mortar as mentioned. the lieg i consider generally better than a mortar and i haven't used or seen the pack howitzer in long enough that i'll avoid commenting on it.
the problem with not having core units up to standard is that the game design assumes that everyone has these standard units and so situation that require them are disproportionately difficult for factions missing core units.
snipers i don't care about; they're not a core unit.
tl;dr: factions can work without the core HMG/mortar/AT gun trio but it makes the faction gimmicky and gimmicks are not good for gameplay. i'd rather have similar, but solid, factions then ones that are frustrating in certain situations; asymmetry is good but only to a certain point.