On the surface Stinking Cloud seems great:
You create a 20-foot-radius sphere […] Each creature that is completely within the cloud at the start of its turn must make a Constitution saving throw against poison. On a failed save, the creature spends its action that turn retching and reeling.
Wow, they can do nothing, if they fail the save!
Except from walking out of the area, and shooting at you, casting a spell, or just hitting one of your allies with a stick. The best you can hope for is they fail their save, and spend their action "retching and reeling" for that turn, but then they just leave the area. Spending an action and a 3rd level spell slot just so maybe make some monsters spend their action seems like a very bad deal. You can't even put it on the big boss once your teammates surrounded it.
- If you put it on top of its head, it just has to walk 20', most monsters can do it easily
- If you want to use it seal an entrance, it only slows the enemies down, but you can not use this slowness to shoot at them, as the area is heavily obscured
- It would work great with Entangle or Web, they need an action to get out of, and the enemy is denied exactly that, but these spells require concentration too
- If you have a 10th level Monk in your party with Purity of Body and Blindsight, he would really be effective in the middle of the cloud, but those are rare.
So how can Stinking Cloud be used effectively?
Best Answer
Forced Movement
The saving throw is triggered at the beginning of a creatures turn. So if you have a way to move creatures into the cloud, they will not have a chance to move out of it before a saving throw is triggered (unless another force moves them out before their turn rolls around). Using this technique you can lock down enemies for longer than usual. Some ways to forcefully move enemies are:
As Fog Cloud+
As you mentioned the spell's area is considered heavily obscured. This means that the spell can be used tactically in much the same way as you would use Fog Cloud.