It all depends on your attitude. If you are happy losing to players to the same skill level, and beating only those with lower skill levels, then Merc dps is viable.
This is what it pretty much breaks down to. If both you and your opponent are at a high skill level, all things being equal, you're going to lose and there's nothing you can do about it (in most cases). If you're okay with that, then the class can be a blast.