In most cases the Retaliation boon is neither of much help or bother. From my experience it doesn't make much difference in PvE, at least not that it really matters, nor would I assume it changes much in PvP.
The problem comes from how it determines and applies its damage and the reason why I create this post comes from WvW and playing a Ele and Rev in that game mode, so this will be focussed on that.
In WvW for most classes Retaliation damage does not matter as it is applied on a per-hit basis instead of taking into account how much damage you dealt, so a class doing 10k damage in one hit will only get one tick of Retaliation, while a class that would do 10k damage in 100 hits of 100 damage would take 100 Retaliation ticks (and probably die).
I love to play both Rev and Ele in WvW, and this difference is massive. Many people would imagine Ele as a hard-hitter instead of a class that wittles down the enemies, but in reality, Staff Ele relies quite a lot on putting down many AEs of rather low damage, instead of, for example, the single big-damage attacks that a Rev throws out. Now an Ele is already an extremely squishy class, even when running full Marauder your health is barely above what some classes have as base health, and armor is pretty much non-existent.
It is already difficult enough to take on a full-buffed out enemy zerg, but Retaliation means for some classes that you can't even attack them without risking your life - without the enemy even targeting you. There is no skill required, no attention needs to be paid by the enemy, no action to be taken. As an Ele, merely attacking the enemy zerg can kill you, without them having to do anything. I literally had that happen several times earlier, where bombing an enemy group, focussed on a keeps lord, not moving out of the bomb, would down me despite doing whatever I could to heal myself. I don't know which other classes suffer such severe problems with aforementioned boon, but overall I think a mechanic which kills you without the enemy needing to do anything, with nothing you can do about it besides "not attacking at all", is horrendously bad designed. The game is already clustered with reflects (and other projectile hate) to the point that playing anything that isn't mainly ground targeted is already an annoyance, but at least reflects can be avoided. Retaliation on the other hand can only be avoided by not attacking at all.
I only really see Retaliation harm those classes that rely on many smaller hits. If you hit few big hits, it can be ignored. It mainly harms those classes, that already are very squishy and need to play carefully, but playing carefully in this case would just mean not attacking at all or having a dedicated healer next to you. For all others it is negligible.
The main problem is, the enemy should not be rewarded for allowing themselves to be hit, just as you should not be punished, without counter and without the enemy having to react, for attacking. This boon works very unfairly against some classes, while being of little consequence agaisnt others.