The target lock can only affect one main ordnance attack from the same squadron. Nothing says that it ignores defensive MARs so yeah, hard target, cloaking etc still apply.
Two Locutas provide the same LR firepower as 3 Lamana or the effective range firepower of a single Sedna a but at 40" for a bargain price of 220 points (3 Lamanas cost 360 points with a range of 32"). And unlike other tank destroyers they are quick (even the Gorgos only has a cruising speed of 7"). Yes, they are fragile, only come in pairs and don't have any cqb worth mentioning. They also have little improvement in ER. All this adds up to: keep them at maximum range, preferably on the flank. 12 da can be expected to put a damage on most targets, especially if you get flank shots.