Apart from WestJet facing recent problems with their WBs, the other example would be Norwegian Air Shuttle expanding into 789s. Overexpansion as a LCC WB didn't end too well.The main carrier who seems to have done OK with adding 787 is WestJet. But even then it has been problematic dealing with competition. Their latest strategy would be akin to VA using widebodies exclusively out of Perth (or at a pinch ADL).
Continuing on the LCC and Widebody theme, Air Asia X and Scoot had problems integrating widebodies into their LCC networks (The later via the merger with TR). The former ended up filing the Malaysian equivalent of voluntary administration/bankruptcy and having to scale back their wide body fleet.
Of course the later is part of the SQ group, thus the parent are able to cover any expenses (not taking into consideration the COVID shutdowns of the past few years).