It's been a couple years, but when I built our in-office render farm for my previous company I also got motherboards with built-in 10G because they needed 4GPU's and there simply no more PCIe slots left. There were so many connectivity issues, but eventually it was solved when we replaced the switches. When I first built the farm there was only one brand that sold cheap 10gbit ethernet switches, but a couple years later finally ubiquiti started making them as well and I think now all of the semi-pro brands sell 10gbit switches. Since we swapped to ubiquiti switches we had no more connectivity issues, not even with the cheap 10G interfaces.

The good intel 10G cards were not expensive at all by the way, I bought them for later additions, and they were cheaper than the premium we paid for the money-gamer motherboards that included 10G cards that I saw you were unhappy about too.