Would be more balanced or safer. Niara's examples does show odd discrepancies (in tabletop 5e), where the Action Surge combo is oddly better than a pure caster :P.
I wouldn't necessarily say it's better. Remember, in order to gain Action Surge, it will cost you two levels. That's two levels of delayed spells and you also end up losing one ASI (which are also delayed). There are definite penalties to multi-classing. I'd say in general, full casters would benefit just staying one class. Fighters on the other hand (as well as other martial classes) may possibly benefit from dipping into other classes. But even in those cases, it's generally best to stay one class.