There are no notable breaks in the balance of the spell system caused by allowing bonus action spells to be cast alongside action spells by the same character in the same turn, with the exception of one notable high level BA spell combination, and sorcerer's quicken spell. No, Rag, the cleric spending their spell slots to cast a healing word and a cure wounds in the same turn is not a balance problem; healing is, until much higher level a generally very ineffective use of actual combat turns
In Larian's game, the smoothest way to make the spellcasting fair and flexible is to leave us with the ability to cast with all parts of our turn economy, without an arbitrary limitation, and to simply rewrite Quicken itself to read:
"Quickened Spell:
When you cast a spell that has a casting time of 1 action, you can spend 2 sorcery points to change the casting time to 1 bonus action for this casting. If you cast a spell as a bonus action using this Metamagic, you can't use an action to cast another spell during the same turn, unless it is a cantrip."
This prevents the quicken spell power break, but still allows player to flexibly cast with their Actions and BAs elsewise - and also allows them to counterspell mid-turn if they need to, regardless of the situation.... which other folk's suggestions of other limitations ("Don't allow the casting of two levelled spells on a turn", etc.) tends to indelicately block out. So, this is the solution I'd like to push for on that score.
