People that state that completely ignore the fact that range has more to do with weapon and that shields give no inherent advantage to being at range. Weapon is the easiest to go with.

"Fight at range; that's where Caldari win!"
This is entirely dependent on a weapon rather than the suit's defensive choice. An Amarr Assault with a Rail Rifle or Scrambler Rifle (they have similar range, yes?) is just as good at long range as a Caldari Assault with a Rail Rifle of Scrambler Rifle. Fighting at range means you are likely getting the first shot (surprise) and your targets may be using a weapon that is incapable of harming you. In those conditions, it often doesn't matter what your defensive stats are. You could be in a naked Scout suit and still probably be in a not terrible spot. It has almost nothing to do with the suit itself; range is all about the weapon.

The fact that one of the only arguments for Shields not being inferior is essentially "fight in areas where the enemy is incapable of hurting you" doesn't bode well. Seriously: what is the difference between an Amarr Assault with X weapon at 50 meters and a Caldari Assault with X weapon at 50 meters? What would happen if they were put at 15 meters?

The next part is the belief that a Shield suit is somehow superior to an Armor suit at range. As stated above, it has more to do with weapon but even then they are probably equivalent. An Amarr Assault can throw out some of their Plates for Repairers and essentially do the "I have less EHP but I regain health fast" shtick of Shields. The Caldari suit doesn't get some special bonus for being far away. Anyone can do it.

It isn't that Shield suits are better at long range compared to Armor; it is that Shield suits are quite bad at close range compared to Armor. At close range, it is more often a DPS race with some variables. Shields, naturally, have lower EHP than Armor so they are worse at close range fighting where shots are a lot more likely to hit. So it becomes a "I am not better at long range, which is where I guess I am supposed to fight, and I am worse than you at close range" sort of deal.

When people say "Play the Caldari right! Fight at range, flank the opponent, and don't just Rambo into the enemy team!" they are basically saying "play better to overcome your inherent weaknesses."
If one side can "play better" and utilize strategy like favorable fights, flanking, and map awareness but can also be Rambo while the other side is essentially relegated to only "playing better", how is that not an obvious weakness? If one side requires X for Y result and the other requires X+2 for Y result, is that not the definition of underpowered?

Think of it this way:
Since the nerfing of the Bumblebee, there have only been two Shield suits that have come to the forefront enough to earn the ire of the forums. The first is the Caldari Scout. It has amazing Shield stats; seriously, ever Caldari suit needs to get base recharge, delay, and depleted like these (I think that is one of the problems). It is also very fast and has the incredibly powerful Cloaking tool as well as having the best overall bonuses.

The second is the reworked/buffed Minmatar Assault dropsuit. It had Scout like speed and still boasted a fairly decent EHP. It was a good suit.

It took two suits that pretty much everyone was in agreement were too powerful, for a Shield suit to be considered decent.