They build up damage too fast and unlike Shadow Spectres they can quickly swap between targets maintaining said DPS. The only counter is to knock them down/stun them to reset the charge but they often get back up to lvl 3 before you can blink again. Worse, they deal bonus damage to skimmers and because there's no distinction between vehicles and heavy infantry in this game they also happen to obliterate units like Wraithguard and Nobs.
This is one of those ill-advised changes Relic made a few months back that has completely fucked up game balance, much like Tac damage buff.
My proposal would be to have Lascannons overheat, much like Plasma Guns do or to significantly increase the amount of time it takes to go from lvl 2 to lvl 3.