You might say the knight is hulking and looming ominously, but does that mean 20 AC, 50 HP, one attack at +6 for 1d8+4… Or does that mean 24 AC, 500 HP, three attacks at 1d8+8 (slashing) +1d4 (negative energy)? Could be either! The range of possibilities is largely unbound and arbitrary.

That's more of a DM/GM description issue than a gaming issue. Like nothing about what you said is specific to DnD, it's just how the person is going to describe the person. But even then I prefer it that way. I don't know what the situation is going to be like until I try to fuck with that. Also, you could just ask to size him up. Insight checks exist, perception checks exist, etc. But I'm kinda pro-having the enemies vague. I loathe video games where I see a number above an enemies head and know whether or not I can defeat them. I'm here to roleplay, not be told immediately whether or not I can take the dude.