This was actually not an issue in Destiny 1 on Hard. In the harder missions that I did over and over, there was almost no safe place that you could retreat to for cover. You might be able to get into cover long enough for your shields to come back IF you did some big attack like a grenade or a super, or were at the end of a wave of enemies. Otherwise, if you just tried to duck for cover in single player, enemies would come around the cover and kill you. There’s some very mobile enemies in the game.
Destiny 2 kind of does have this problem, potentially, because that heal move they added in Destiny 2 to the character we were playing allowed him to take cover and heal even if he’s being hit.