Not sending engineers, but when I worked for a smaller dev studio we did need to contact Unity and get support a couple times when we got stuck on something. I think we ended up paying for access to the source code and the development manager ended up making a couple of changes in the source code to get something working better as well (I don't remember the specifics, I was working in Unity but on a different game at the time).
This was like 10 years ago though. That guy worked on multiple arcade games in Assembly back in the day too (you've likely played games he worked on if you've played more than a handful of popular arcade games), he was no slouch.
How often do you think that happens for unity? _Maybe_ for some massive developers, but I don't think this would be very common for most customers.