I think the previous post stoopicus and Lotsanerv pretty much explained why merc is broken: op is just saying the truth about merc having no way to stand against getting focused before, without taking into consideration that that was their only weakness. Lethal weakness? probably, but now that they are cured from that you have a ranged class with so many abilities, excellent difference and the highest focused damage class in the whole game.
Its kind of funny, everytime Bioware tries to fix something, they create a bigger monster