Much of what makes the Wolfhound excel against other light mechs is its armor. It has the armor to absorb the bombardment they throw at it, whilst they do not have the same in return. To obtain this edge, it makes sacrifices, such as having a minimal speed for its weight class, or relying on distinct brackets that are not impressive for its weight class. It is quite the downgrade from the PHX, as it lacks the mobility, or machineguns that give it options against all kinds of foes. This is one reason why it loses to stock Jenners in many duels. The armor difference is the defining characteristic of the Wolfhound against most foes that it can overcome.
The Mercury is not a foe that the Wolfhound wants to fight, even if it is far from defenseless in said fight. With 4/5 pilots, standard gunnery/piloting skills, you are not going to get much from the Wolfhound's main gun, relying more on armor than anything else to make it useful in a generic fight. One of the reasons that the Large Laser is not a common main gun for lights is that they cannot layer on it like Mediums and heavies, but have to make considerable sacrifices for it. Furthermore, the Mercury is an aggressive mech with similar armor, but with optimized heat efficiency. The Hermes II Mercury may itself lack mobility, but the 2 mp running difference will rarely give it any issues in closing, and grants it a greater edge when it wins the initiative than the Wolfhound can get in similar situations. The initiative edge can be mitigated by a series of multiple losses, but that is true of many respected mechs.