The verdict

CPU
Beelink SER9 has 1.2x the CPU throughput (34,991 vs 29,364 PassMark).
Idle power
Beelink SER9 idles lower (8W vs 9W) - cheaper to run 24/7.
Networking
Minisforum UM890 Pro has more NICs (2 vs 1) - better for a firewall or VLAN trunk.
RAM ceiling
Minisforum UM890 Pro takes more RAM (96GB vs 32GB).

Which wins for each job

Job Beelink SER9 Minisforum UM890 Pro
Plex / Jellyfin ~5 streams ~5 streams
Frigate NVR add a detector add a detector
Proxmox / VMs 34,991 PassMark win 29,364 PassMark
Firewall / router 1 NICs 2 NICs win
Local AI / LLM no slot no slot

Approximate - capacity estimates from curated specs, the same logic as the recommender. Verify against your exact setup. Prices move; check the retailer.

Full specs

Spec Beelink SER9 Minisforum UM890 Pro
Price (list) $999 $479
CPU AMD Ryzen AI 9 HX 370 AMD Ryzen 9 8945HS
Cores / threads 12 8
PassMark 34,991 29,364
iGPU Radeon 890M (RDNA3.5) Radeon 780M (RDNA3)
NICs 1 2
NIC model Intel I226-V Realtek RTL8125
RAM max (GB) 32 96
M.2 free 1 1
PCIe slot none none
Idle (W) 8 9
TDP (W) 65 54