I've had Instacart workers make pretty good substitutions that weren't even in the app's automated list of replacements. They marked my original item OOS and added the new items as an "adjustment", so these judgement calls are at least something that is possible for them to do.
The issue is that YMMV with the worker you get on any particular occasion.
To clarify, my negative experiences with Instacart were restricted to the shopping website/app experience (which is no better or worse than any other); the Instacart shoppers have always made intelligent decisions on the ground.
The issue is that YMMV with the worker you get on any particular occasion.