A friend and me killed the level 72 boss in world level 4 2 times and both times it dropped 6 forgotten souls. So if you got the damage it is absolutely worth it
We do know the helltide spawn pattern. They're up for 1 hour, and then when it dissipates, it spawns again after 1 hour and 15 minutes. This cycle continues. Also, mystery chests, while nice, are definitely not the most "consistent" way to farm the forgotten souls - the best way is to hunt for the "helltide commander" elite mobs that always drop some souls or roses (or both if you're lucky) and the screaming ore veins. There's roughly 15 ore veins per helltide, so a good tip for efficient forgotten soul farming is to simply do a quick ore check every time one is up, takes about 5 minutes to run around most edges, depending on how many mobs you are able to skip
The Spawn Rotation doesn't make logical sense, and seems to just be a way to push metrics and have people log in to get them. Helltides are boring and not-fun at all. The fact they are time gated and required is just bad. I've done 3 Helltides. I've walked away with 6 legendary items total across over 10 chests. That is a VERY bad rate, and some ^&*!ty RNG/Lucky. It makes me want to just avoid them completely as a waste of time.