So, my understanding is: Installer does not crash because nokogiri is not in inst-sys, but it may crash if the user selects nokogiri for installation AND uses second-stage (only AutoYaST uses second-stage now). Also Firstboot may crash (if nokogiri is installed). The question is if nokogiri is actually supported/available for SLE 12 SP3? Anyway, all cases can be fixed in MU.