It’s not a critical flight control surface, it’s a secondary control surface that adds extra lift at the cost of efficiency. Lightning is not inherently dangerous to airplanes. They are struck all the time, and it is fine besides alarming passengers and occasionally causing some minor concerns and repairs. Composite aircraft like the 787 need significant additional lightning protection though, this is a known risk for them, and Boeing intentionally decided to decline to lightning protect non-essential areas of the aircraft despite the potential for lightning damage, and that is a perfectly safe albeit probably financially and reputationally stupid decision.

For the aircraft, having slats stuck in either position is obviously not great and not having it available potentially limits the available landing options but it is not a safety issue. Efficiency concerns may result in the flight being unable to continue, but again, not a safety issue as they carry extra reserve fuel for unplanned contingencies like this and have alternate airports available to land at anywhere along their route. As a mechanical issue where a single incident cost the airline in question hundreds of thousands of dollars in revenue and delays and shuffling around other aircraft and flights to cover for the loss, it will be investigated and addressed if possible. But nothing unsafe happened here, except to ANA’s revenue. Inconvenient, frustrating, maybe even alarming, but not unsafe.