There’s that old line that if my aunt had wheels she’d be a bicycle. Maybe the command form is muddling the topic here, but using the be-verb with an adjective like that attaches a subject complement, essentially describing the subject. But “I am fast” describing a person doesn’t mean that saying “I drive fast” is describing a drive as a noun.
I don’t even see “nice” in “play nice” as an adverb. You could switch “play” for “be” – “be nice”, same with “be safe”.
Playly not nice.
There’s that old line that if my aunt had wheels she’d be a bicycle. Maybe the command form is muddling the topic here, but using the be-verb with an adjective like that attaches a subject complement, essentially describing the subject. But “I am fast” describing a person doesn’t mean that saying “I drive fast” is describing a drive as a noun.