Myself I grew bored of the Mekwars campaign very quick. The problems to me were:
a) relatively low number of players, slow battles because everything from moving to targeting is done player by player - in other campaigns even 15,000 BV vs. 15,000 large map battle may only take 50 min...
b) 3025 - not a lot of unit variation
c) force size modifiers and unit type rules used... So you dont want to use an army bigger than a lance usually and theres no slow meks, little to no vehicles, no BA etc.
d) the match making is not random - many veterans pick on the newbies, stalking them, even using "bait and switch" to trap players
f) the repair system, unit pull system, everything just sucks in it
g) you cant even move pilots form unit to another, and thanks to the other rules you want to get rid of most LEVELED UP units asap - and do that by garbage-dumping them onto the faction pool.
HOWEVER there are at least 2 fully dynamic MegaMek campaigns with the same (heavily modified to their own uses) mekwars clients that have all of those parts fixed. Only things that I'm not really a fan of in the others are the occasional IS vs. Clan battles and the c3(with ECCM spotters...).
The above is of course my relatively biased opinion which is effected by the fact that I got abused by the admins there. Battletech online campaigns played on MegaMek can be GREAT fun at their best!