I see this a moderate amount in fintech, or perhaps at least older fintech?
Had a colleague tell me about a new financial info provider he was supposed to connect with for data. "Oh this one will be easy" he was told. "They use oauth". OK... well... you oauth in, then you hit an endpoint that triggers a data build which then puts your data file in to an FTP account of your configuration. But you can't even configure it via hitting the endpoint behind the oauth. You have to log in to a web app to configure the FTP account for your account.
You hit the API endpoint and get 'OK'. But... that's it. You might get a file in your FTP account 5 minutes later. Or an hour later. It's just whenever it gets processed on their end. And... if it doesn't show up - you have no indication whether something broke, or perhaps the data service "just doesn't have any data for you today - you can't expect data every single day".
It's insanely crazy how across-the-board poor so many of these mid-level data vendors are. Colleague is currently supporting 8 different integration ingests like this - some deliver FTP daily, some you have to pull down, etc.
One delivers data which is only ever a delta. You have to continually request previous daily info from them to get to the 'first record' for that account, which can sometimes go back years, but... you don't know that up front. So you have to loop and request "day before", pull down data, parse it, then check the day before, until you don't get anything for that customer, then presume the previous was "initial balance". But you also can't just do that once, because... there may not have been any data the day before because... holiday? system down? So in practice you need to go back 3-4 days at least to verify. Oh... and don't keep looping and requesting too fast... you'll exceed the low rate limit ("we don't want people abusing our API").
I can only barely imagine how systems evolve like this, because they don't really seem to serve anyone's interest fully.