My first zigbee device was Sylvania (73685) rgbw led strip. On it’s own it worked fine but once I started trying to add sengled zigbee bulbs I kept having problems with pairing the bulbs or keeping them connected. Zigbee uses 2.4ghz so I thought maybe interference so lowered power output on wifi AP 2.4ghz band & even turned it off for testing but didn’t help. I think I read somewhere that sylvanias caused that problem. I unplugged sylvania led strip, reset & rejoined everything else and had zero problems. Tried to add the sylvania back in and disconnect problems came back after awhile. It was past the return period so now I keep it unjoined and plugged into a zigbee smart outlet. I sometimes have to remove power from it to join new devices or when I restart zigbee controller. Currently have mix of sengled bulbs, thirdreality plugs, sonoff plugs & sensors, and single aqara tvoc all working together fairly reliably.
You can try seeing if you can set the speed/duplex of NIC/ports manually if auto-detection keeps getting it wrong.
Unifi I like the APs for mesh & multiple SSID+vlans but I keep them on dedicated vlan with zero internet access because I don’t trust that I properly followed instructions to disable opted in analytics/telemetry. The mgmt software is alright but new UI wastes a lot of space. The PoE switch was alright until it stopped being able to keep a config last year. USG router I kept less than a year because it was too slow with any useful features enabled. I’ve glanced around at replacement APs here & there but pretty much waiting until I have more wifi 7 compatible devices and that’ll be another couple years.