I figured out why the guns are so bad.

I almost chocked to death on the straw that I'm chewing on.

Good job SIR!

I think it is generally pretty obvious that game developers nowadays don't play their own games, hence the "debatable" balance changes (exceptions may exist, of course). Look at The Division, look at SW:TOR, look at ME:A, hell look at No Mans Sky! Chris Roberts is no better with his "buy imbalanced nonexisting ships for cash!"-Financing model.

To be fair, the Krogan heavy melee in ME3 often made you slide around the enemy you tried to backhand if your aim wasn't spot on, which could result in getting shot in the back by that same enemy. Most of the normal heavy melee attacks like the Quarians' or Salarians' etc. didn't have that much "hit feedback" either, so that doesn't bother me as much in ME:A.

That they didn't like people using only one weapon when they gave them the possibility to take two is a valid opinion - their solution to that was...well...strange.

Also I think we mustn't forget that BSNers are clearly the elite of MEMP players with a skill-level 90% of players will never achieve. Just look at what types we had to deal with (and still have to) in ME3MP. Totally clueless people with no concept of teamplay, ignoring teammates, objectives, spawn-nuking on extraction while those rockets were direly needed in the objective waves etc.
The game was clearly not designed for the top 10% of players, that's why we "can" appreciate when they at least listen to some of our constructive feedback, but we shouldn't assume that they'll change everything because of us.

It sucks, I know - we'll just have to live with it and see it as a challenge (well, sort of). Or just move on to a new game and forget about this. 

That's what the loading screens say in MEA but I don't believe it is actually true.

Haven't really tested it, mind you. I'm just going off 'feel' -- it doesn't 'feel' like SMGs do more damage to shields nor that single-shot pistols do more damage to armor.

I don't know that I would call it wrong.  Hard hitting weapons suffered from shield gate, as you stated, but high ROF low damage weapons suffered a higher penalty versus armour too.  If memory serves it was a reduction of about 50 damage per round with a minimum damage value of 5.  Those numbers could be off though.  It's been a while.