My Millies lock up great, very trustworthy and strong locks on those guys. The only concern might be when doing a hard twisting cut getting your hand in the cutout and accidentally disengaging it, but I have never had that issue, and the cutout really helps to disengage the knife when you have gloves on. My Barong lock up perfectly, doesn't have the handle cutout like the Millie (this makes disengaging the lock harder, especially with gloves, but arguably makes it less susceptable to accidental disengagement), but as you can see from Yab's pics it isn't as HD as the Millie. Considering the 3mm blade in the Barong Vs. the 4mm blade in the Millie I don't think there is anything to fault on the Barong's lock strength. It is plenty strong IMO for medium duty use, and with the Krein regrind I have on mine it is an unreal slicer for a 4" blade. I prefer lock backs, Ball Bearing locks, or especially the comp lock to the liner lock, but Spyderco certainly does a liner lock right.
Liner locks are easy to do wrong, as I found out when sharpening a knife my Mother in law paid $45 for at a local knife shop. Keep in mind this was a brand new knife that was a bit dull from the factory. With minimal pressure applied to the spine during sharpening it was disengaging on me. That is not a good feeling! I tightened the pivot as much as I could and had to tighten the bottom handle screw (the only 2 points holding that thing together) and told her to use it like a slip joint if she must use it, and that even a slip joint is safer. That knife has a flat tang contact and just a bit of side torque is all it takes to disengage it. Luckily with my Spyderco liner locks they engage solid with no play and the radiused tang seems to be a great way to build a liner lock. They are built strong and built to last.
Mike