Hitting bumps on a locked out suspension fork
I have read about damaged lockout systems from big hits while locked, so my advise is treat a locked fork as a fragile fork unless you want an expensive repair bill. Probably the key point is what is "a hard hit" and "high load"  for a fork? That said, I have taken a couple of what I would call big hits with my fork locked with no damage, but I weigh under 70kg and what I call big is probably nothing to some people. 

The manual for the Suntour forks is somewhat contradictory. In one line it says "...This system protects you when riding in demanding terrain in case you forgot to unlock the fork." 

Then immediately it states
"...The fork may become damaged when it is compressed under a high load...." 

The take away I get is it protects you but will possibly be damaged. In your case, I would suggest cranking up the pre-load when on the road, and only using the lockout when you know there are no potholes around.