We have a mix of about 30 B5/B5C devices plugged into Routerboard 1100AHx2 and Routerboard CCR1009 and Netonix switches. I havent noticed any eth issues on any of them.
We always use shielded outdoor cat5e or shielded outdoor cat6 and have had good success so far with the Platinum Tools ezrj45 shielded connectors.
We have had many problems with UBNT toughcable shielded ends over the years… are you using these ends?
Well there is one link that with a B5C the eth only locks at 100mbps to a netonix switch, and I have let it run like that for months until i visit the site again, but im sure its because it was installed with very old cable and ends because it was just replacing a Mikrotik radio and is used for spectrum analyzer only so far. It doesnt flap.