Camera and camera lens names. I tried mellisearch (1-2 years ago), and while I loved the simplicity (I barely understand what I threw together with many, many lines of C# code for elastic search; this is partially on ES, but clearly on me as well), it was not good at getting results.
Names like "Lumix DC-S1IIE", "DSC-RX100 VIIA", or "FE 50-150 mm F2 GM (SEL50150GM)" do not quite work with default tokenizers and analyzers. Of course that is for product names, for full text queries still need to use normal language rules… except for product names showing up in the text, so now I need multiple indexes for every field, and searching different sub-fields sometimes with different query analyzers.
It was a lot of trial and error getting ES to both find what was searched for, but also be typo tolerant. It’s very easy getting far too many results, and bad scoring for fuzzy queries.
So a bit of a special case, but something the customization capabilities of ES support pretty well.
Luckily, our dataset is rather small, maybe 100k documents, so scalability is not a problem.