The "Ineffective" is as you thought, an indication that the enemies armour is deflecting most of your weapons damage, however I have sene the same particle explosion so perhapos these resistances have vertain limits where they can completely ignore damage and the n after that they take a reduced damage level?
The Gatling guns at this point are one of the most overpowered weapons in the game, the higher power gatlings have the same damage as the higher end rifles, weigh roughly the same and have similar ranges yet fire massively faster, this barrage of mid range damage (resistances or not) devastates most enemies online or off. So yes they are worth it, although I use them begrudlingly.
Beware however we are currently on the 1.01 version of the game, the japanese servers have recently moved on to the 1.03 version where there are a number of changes regarding weapons that may well change the balance of the game.
Personally I'm a fan of the high power, high capacity laser rifles, noone seems to have high TE defense because of the popularity of KE gatlings and CE battle rifles.
The Old Gods
Never left this Place