Here are some general issues that I found out so far. Casting two spells per turn: If you cast two spells during the same turn (i.e. one on the action and one on the bonus action), according to PHB, at least one of them has to be a cantrip. It is not possible to cast two leveled spells on the same turn unless special rules apply.
Attunement: During the game, a lot of items can be found that give special advantages like free extra spells. These items can be swapped among characters so that all of them can use those extra spells. But normally these items require attunement during a short rest before they can be used. Furthermore, there is a limit of how many items can be attuned at once.
Eldritch Invocations: Just like spells, Warlocks can swap Eldritch Invocations for another if they level up. However, switching Invocations during level up is not possible as I experienced. You can only choose two invocations when yu reach level 2 and that's it.
Inventory: You can very easily switch items among characters, even if they stand very far away from each other or during fight. Normally this requires the two characters to stand in adjacent spaces and a bonus action during combat.
Dodge action: There is no possibility to take a dodge action to give the enemy a disanvantage on attacks. Very crucial for cast empty wizards and sorcerers.
Resting: The party can only make one short rest and then has to take a long rest. According to PHB, there can only be one long rest per day, but there is no limitation to short rests. There should really be more opportunities to make a short rest.
There might be more issues but those are the ones I immediately noticed.