While there are a few exceptions out there- as such unit exceptions exist in all wargaming systems, the allocation of battle value for mechs between opponents works well enough in the game for a "fair" match. We both agree how much BV we want to play- pick our mechs and support units- tanks, infantry, etc. and roll for a mission.
On average for the games I play and Battletech gamers I play with we can do about 5-6K in battle value in about 4-5 hours playing at a normal game pace. Are you using battle value in your games? Are there any alternatives? Early on in my Battletech games we would use a mech for mech build- usually centered around a lone lance without vehicle or infantry support. Pick one assault, one heavy, one medium, and one light, or two heavies, one medium, one light, etc.
Not as "fair" as battle value, but quicker to pick mechs, it worked most of the time- occasionally picking a dog of a mech or a mismatched mech. Finally way to explore the game was using a scenario or game master type setup.
One players is very unbalanced, but it relates to the mission...
A medium mech vs. lots of heavy mechs with the medium mech having to make it off the table. A heavy mech against multiple heavy mechs trying to see how many turns it can last.
Narrative vs. fair "battle value".