If the original OPN feedback manager doesn’t work for your left ear, but the new OpenSound Optimizer feedback technology of the OPN S works for your left ear, then I think the occasional warbling is worth putting up with in order to be able to tame the feedback for your left ear.
While the original OPN feedback manager is nothing new and rather mediocre, it’s probably not much worse than other HA brands’ feedback manager. They really all just use the traditional 3 prong strategies of phase change, frequency shifting and gain/headroom reduction. There’s a good chance that you will run into the same feedback issue in your left ear with other brands, assuming that you’ve already used the proper fitting (like with custom molds) to reduce the other variables.
If the OPN S OpenSound Optimizer has solved the feedback issue on your left ear (albeit not a perfect solution yet due to the warbling), if I were you, I’d stick with it because the occasional warbling is the lesser evil compared to the evil of not being able to solve the feedback issue on your left ear.
And hopefully one of these days Oticon will solve the warbling issue for you. But for now at least they’ve solved the feedback issue on the left ear for you with the OSO.