HFSCORE is wrong, anything that's not running current major version, i.e. 1.28.x is just wrong (because such witnesses are effectively not producing, you can score minor versions though). Disabled witness or one who is missing blocks is just no go. Price feed is good if it's accurate and no older than 24h. It can be few hours old but as long as it's accurate there's no point changing that.
I disagree that out of 20 witnesses' price feed is pointless - backup witnesses are no real backup if they can't do what is expected from a witness.
There's also things that are harder to do metrics - if a witness is running a seed node? Or supporting the infrastructure otherwise? Their stake in the game? That's not easy either because not all witnesses have their HP on the same account as the block producing one, etc.