I've been in the database world for a very long time, think before relational databases were invented.
How many simultaneous connections do/will you have to the DB(s)? If not many (< 10) then faster clock speed and cores will help, but only marginally. Cores come into play with many hundreds connections to the database.
The ruling party in database sizing is RAM ... the more the merrier.
Basically the reason comes down to how much data can be cached in memory. ECC RAM is better and 32-64 GB is a great starting place.
Your choice of OS will matter too.
As far as storage, I recommend that you store your schema's, tables, and their data in an M.2. that is not your boot drive.
If you are creating these databases for hobbyist reasons, and not needing Error Correction memory (ECC), then the NUCs are a great platform.
I don't know all of the supported NUC memory types per NUC, and suggest that if you are planning to store real data that is business related, whatever platform you get must support ECC memory.