I'm refering to WoL's 光は我らとともに (
Hikari wa warera to tomoni/' The light is with us'), which covers AoE attacks. Given the frequency of AoE attacks it's somewhat of a crucial difference.
Neither WoL nor Wilhelm can cover magic attacks, but for that you have SPR buffs (Ramza/Roy) and elemental resistance, and certainly Mystair as you mentioned. Though you can also give Wilhelm a benefit of his General Command in this category.
As far as damage mitigation, WKN also already has 75% damage mitigation and 100% provoke (you can already solo Bahamut Trail with him), but all of that is pretty much covered with reraise anyway.
The thing about 5*s is that they need to be a ever-so-slightly unbalanced as it really costs hundreds of dollars statistically to summon one, and this really isn't a pvp game with the shallow arena system. Wilhelm giving a broken advantage in clearing content is to be expected, but the existence of WoL lowers the value proposition in those lucky enough (or pay enough) to pull him.
Given Rem, Ace, Queen, and others have been buffed for GL, having a Wilhelm buff seems fairly reasonable; perhaps getting the some the ability enhancements from the start. The WoL delay may also signal that Gumi are cognizant of this.
