They have yet to hit a happy medium....
The Xmas event.. REQUIRED grinding for the possibility of all the characters... You could not purchase frags past a certain amount. If you were lucky, you might have gotten all the characters from the frag purchases. Or subsequent souls garnered from grinding. You were guaranteed at least 1 of every frag option if you purchased all the frags you could.. so the highlighted Eddie was a guarantee if you purchased enough. But there was absolutely no guarantee anyone would get any other character. The guarantee was only for ONE character in each frag set.. and since frag sets contained more then one character.. it was random. I didn't get the gunner ghost. Nothing I can do about that. After two weeks of grinding and 4 tries with the frags.. I gave up.
The Dark Omen even REQUIRES purchase (most likely) to get all the characters. But it also guarantees all the characters at a certain purchase amount (typically around $200 US).
What they need.. is the Xmas event style with changes.... Allow purchase to a point, and then one must grind.... BUT.. and this is IMPORTANT... STOP putting multiple event characters in the same frags. It is nothing short of infuriating to grind for two weeks and see the same character drop repeatedly out of a poole of ONLY 2 characters. That's NOT a 50/50 chance. It's borderline abusive. Either split the characters into their own frags.. or implement some sort of mechanism to toggle which character drops... if Character A dropped last time for UserX, then Character B should drop next for UserX.