The Viks are a crew with an answer for everything, because mercenaries are designed to be answers for things a faction is lacking. Not having to pay the mercenary fee is the only thing that makes the crew workable, since the 0 cache can be brutal at times. The tried and true 25SS Vik+Gunslinger+3xRonin list would have a cache of 1 if we had to pay the mercenary fees. I think the problem with that versatility is that when making a crew you may go against taking a niche model to beat certain masters for a more versatile model in the event they don't take that master.
Sue is the biggest example of this since there are some matches where he'd really shine. However it involves giving up the Trapper, the Gunslinger, and VonSchill, three models which are extremely useful in almost any circumstance. The other problem with mercenaries is that they are definitely a low model count crew which can sometimes be a disadvantage. We only have two options (currently) under 5 stones, one of which leaves halfway through the game. About the only other reason I could think why you don't see mercenary crews topping tournaments is because we play "fair". Merc crews don't exactly warp games or have shenanigans like Dreamer, Hamelin, etc.