These ~1L sized computers idle in the 7-13W range. Even with EU electricity prices, it would take you a very, very long time to make up the difference from a 5W SBC.
You also get standard x86 support, normal expand-ability, M.2 slots, PCIe ports, etc. You lose GPIO support.
For $100-$120 on eBay, a search for 'dell optiplex 3070' typically results in getting either a i3-9100 or i5-9500, 8-16GB memory, and 120-256GB NVMe. I bought one a couple months ago for Blue Iris and it uses 8W at idle. Here's an example of a USFF (ultra small form factor) for $110 - https://www.ebay.com/itm/325826958401
Lenovo variant (M720q tiny) for $121 w/ i5-8400T, 16GB, and 256GB SSD (probably NVMe) - https://www.ebay.com/itm/266450621632