• Tags and relations are a total mess. There are many different and overlapping schemes. There is obviously outdated/sub-standard data, but automated fixes are forbidden. There are some neat tools to make sense of the tag data, but ultimately I had to deal with all that mess and complexity on my end.
• The lack of moderation or a way to officially act as an intermediary for edits (other than clunky oAuth flow) meant I couldn't expose an Add/Edit/Fix functionality for my users. Similarly, if I wanted to let my users upload a photo of a venue, there was no service to store it, no identifier or tag to associate it.
• The API returns only nodes entirely within an area, rather than everything intersecting a given area. If you fetch small lat/long area, you won't get a node for the country/city/district it's in, unless you stumble upon a point in its border or label. That meant I couldn't fetch things from OSM API as needed, and had to have my own copy of the data and the DB and custom search.
• The map looks ugly without buildings. There was free building data available for areas I was interested in, but I was told to copy them by hand.