Unified memory means faster execution, but it also means that memory will be used for both graphics and generic workloads.
So if you believe you will use heavy graphics, multiple monitors, or (gasp) virtualized machines.
16GB should be the standard.
If you are just a plain jane/joe user, who powers it up and uses it and nothing more 8GB should be fine.
Any MAC user who doesn't have a hardware fault, will try to use their system for 4-6 years. Locking into 8GB may be ok, but my guess is the person that can survive that is in the same minority as one that runs hundreds of containers, a virtualized machine etc.
Even android phones are coming up to 8GB ...