Classes have different difficulty levels, a bad merc will beat a bad anything, but a good merc will get stomped by a good anything (Or almost anything)
Mercs are in no way overpowered, they don't do the most damage out of classes, they don't have as many surival tools as many classes and they don't have as much burst as some other classes, they are just easier to play, so bad mercs (Yes only bad mercs spam tracer) can beat bad other classes because of the class difficulty, but that doesn't make them overpowered... You have the tools to beat mercs you just choose not to use them.
Games aren't balanced around the lowest end of the skill pool, not when the higher end has no problems with them anyway.