Multiclassing has rarely been a problem in 5e that I'm aware of, with the only serious exception I can think of being the coffeelock (Warlock that turns their spells into sorcery points, short-rests, accumulates more warlock slots, more points, ad infinitum, resulting in endless points and in some cases dipping into paladin for arbitrarily huge smites.) Even if there are issues, multiclassing takes both patience and significant game knowledge to optimize like that, and unless there is a competitive aspect of the game against other players, what would it matter if there are some broken combos? with the element of surprise you can kill everything with enough explosive barrels or other cheesy tactics anyway. You don't have to use them, and multiclassing offers a ton of fun for both theory crafting new interesting builds and new interesting character concepts.
