My biggest gripes with them include what you just mentioned.
I usually order from them when I'm making a large meal with typically uncommon ingredients. Today I'm making Cuban — I needed Seville oranges for mojo, yuca, rice paper for turrón, etc. Instacart is a gamble for me, and the UI is frustrating.
I want to search for several items across stores and see which ones have them all. Instead I have to try stores one by one and see what's in stock. It's not enough to get me to stop using them, but it is annoying. But the really frustrating thing is when they claim a store has an item, but the shopper doesn't have much incentive to spend time looking for those hard-to-find but critical items. So lots of my orders have missing items which I then have to go out and find myself anyway.
Also bad is that many items are hard to order specific quantities of. Some common produce items are sold by the pound, when I really just need one two count (e.g., onions). Worse, some items are labeled "per unit" but when I order two, they're converted to a weight value and I get fewer than I ordered. Again necessitating a trip to the store.
I acknowledge that most people aren't making the kinds of meals I'm doing, and not suggesting they cater to my admittedly small market segment. But if they solved these problems, they would be amazing instead of "slightly worth it".