It’s a completely different device. One is a combination of a souped-up USG, UCK Gen1, UAP-FlexHD and a four-port switch and the other is a completely new router with a 10Gbps routing capacity, an 8-port switch, a UCK Gen2+ without the hard drive. The UDM has been out for almost a year and it was in Beta for almost a year before that. They launched it with a few issues but in general those issues were resolved by Firmware 1.5.x which is what the UDM-Pro launched with. The UDM Pro is effectively still only fully stable with Beta firmware although the current 1.8.x firmware is OK for most home users. The UDM-Pro is a cluster-**** of epic proportions and the newer (less feature-rich) UXG-Pro will take over once it’s launched [soon] and the UDM-Pro will be killed off quietly like the USG-8-XG and the UniFi Server XG and many other UniFi devices that never quite made the cut.
As a good rule of thumb, NEVER buy anything a UniFi for 6 months after launch. Let someone else do the beta-testing and you’ll probably be OK. But the UDM-Pro is a DOG of a router. Just scratch it off your shopping list and get something else. You’ll thank me.