Multiclassing will probably cause a lot of issues with the current systems to surface. Right now summoning is kinda borked so IDK how I am going to do my Warlock and Conjuration mage build. Reaction based spells are going to get to be an issue and then monk reactions with a class that has reaction spells might cause things to not work under the current automatic system. And the paladin's smite with other melee attack abilities might not properly work or might be implemented wrongly. Pop up boxes really might be the best way to faithfully translate 5e