Even the non-spell casting multi-class characters suffer from it though. I’ve seen a number of campaigns where Barbarian-Rogues and Fighter-Rogues were cursing themselves for multi-classing as the campaign got into the higher levels. The simple and undeniable point is that in D&D, an ad hoc character will never be as good at fulfilling what the designers intended. A fighter is a tank on the battlefield. A fighter-rogue, a barbarian-ranger, or any other combination might be close, but none will be as good at fulfilling that job in later levels.