Another problem with deviating from 5e rules regarding Surfaces (as they are currently implemented):
Surface effects in 5e almost never cause guaranteed damage without having a significant cost or a significant drawback. In fact, 'guaranteed' damage is relatively rare in 5e without having a costly limited resource like a Spell Slot or Item Charge. They also often have several requirements like 'ending' your turn in it.
Why?
Because (in part) Concentration is a major part of the balance of 5e. The Concentration rule is the greatest limiting factor in controlling spellcasters from completely dominating combat at higher levels like they did in prior D&D editions. It's why a Wizard can't fly around while Invisible and Hasted (at least not without multiple party members working together or exceedingly rare magic items) tossing Fireballs while basically being immune to melee damage. It's a great mechanic for balancing spellcasting vs martial classes...
*HOWEVER* Concentration can be dropped by taking just 1 point of damage. This makes anything that causes guaranteed damage to be *very* powerful against spellcasters (including Gish melee types). This is why surface affects should never, ever cause guaranteed damage unless they do so by the rules of 5e (which as mentioned, were written with that balance in mind).
Surface affects also should *never* cause guaranteed status effects unless in the rules of 5e (Acid causing -2 AC is insanely stupid), but that's for different balance reasons.
