There is no way to generalize how the target AoE’s do in each situation because each situation is determined by the players involved. No compensation is needed for “missing ticks”, because it is the player’s responsibility to line up their slows and crowd control, and communicate with teammates to do the same. That adds a skill-based element to the game. The abilities are not remotely bad. In Mythic Keystones, as I said with Death and Decay, all of the targeted AoE abilities will easily be first or second place on the damage meter if they’re intelligently lined up during mob pulls. The same goes for mass PvP. If it’s just an arena or a duel, then these abilities are better for removing stealth, keeping targets in combat, adding pressure damage to low hp targets etc.
One point that does make sense is the concern of overlapping damage dealing since we already have Remorseless Winter. Honestly, I would have no problem if a preexisting talent was replaced with one that allows Frost to swap Remorseless Winter for a Frost-integrated version of Death and Decay, because I can think of several talents that might as well not even exist in this spec. Regardless, Frost needs a lot of work done to its core concept right now, so the last thing I’m going to do is complain about some Unholy flavor being added to it.
Your concern over the damage overlap is actually objectively correct. The point about the abilities being bad is demonstrably wrong.