In an open sandbox featuring strategic level control in the hands of the players, the DM isn't really creating specific encounters. The players choose their options, which may or may not lead to combat encounters. The players get to do the majority of determining the nature of combat engagement. With this capability comes the majority of the responsibility of what happens to the party.
Surprises are an exception, but a careful group can take steps to minimize their exposure to ambushes. There is much less weight on the DM in such a campaign which is why I prefer them.
When players have more control over the events that can lead to PC deaths then there is more acceptance of the harsher consequences arising from those deaths.
There is no difference between the two types of encounters.
Bottom line: they are both encounters. With the same creatures and scenario, which can happen, they both have the exact same amount of risk.
The rules do not change, just because the DM has a different path to an encounter or that he is creating the encounter on the fly instead of ahead of time. Your point here is illusory. You are convincing yourself that there is a real difference when the difference is moot.
A surprise round is just as risky with the same PCs with the same capabilities and the same NPCs. And in 5E, surprise can be very deadly, especially at low levels. Even at high level, a tough encounter can be very risky with surprise.
Now if you are hand hold your players because of your DMing style, then sure, you can give them more of a chance to survive. For example, instead of flying in and using a breathe weapon during a surprise round, the dragon talks to the PCs first. But shy of specific DM decision making, encounters are encounters. If the PCs decide to get into a fight, the DM should not punish them because of a PC death.
At least from my POV, I enjoy combat encounters. No doubt about it, one does not have to get into combat every single time, but doing so should not be punished with houserules.