The balance problem is not just being able to alpha strike with spells.
It's surprise attacks giving you one full turn too many.
It's Fighters doing Action Surges with torch-dipped weapons. Paladins emptying all their smite spell slots and using another smite with Pommel Strike.
It's dropping Haste and Invisibility potions around like candy.
It's not allowing enemies to roll Perception against stealthy or invisible PC's.
It's creating Magic Items that grant extra Bonus Actions you can use to off-hand attack with a torch-dipped Hand Crossbow standing in a pool of poison wearing Broodmother's Revenge.
And finally, it's casting three Fireballs before initiative even kicks in. 1 Initial 2 surprise 3 haste.
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It's all of the above that really snowballs and underlines the weakness of a turn-based system. If they want to have a good game, they should work to downplay the alpha strikes that make even the hardest encounters redundant.
I would allow a Perception check against stealth or invisibility when attack intent is declared, or simply roll Initiative if the attacker can be seen. A mage who casts a Fireball has to speak and conjure the fire before the spell is launched. That's plenty of time for a high Perception quick reflex enemy to notice and react by spreading out or even firing an arrow. Likewise someone drawing a Greatsword and approaching to striking distance shouldn't really "surprise" you at all.
Other stuff:
Fix Haste so it grants an extra attack instead of an extra full action. And no extra spells for obvious reasons.
Stop with the extra Bonus Actions from items and fire dips that don't cost any consumable items.
