Is Please be advised an imperative passive, or rather advised is an adjective?
Please be cautious/aware/vigilant - adjectives. They can all be preceded by "very" but the passive agent, e.g. "by me" cannot be inserted.
Please be very cautious/aware/vigilant
*Please be cautious/aware/vigilant by me
Please be advised - passive form, past participle. "Very" cannot be added and passive agent, e.g. "by me" can be inserted.
*Please be very advised
Please be advised by me.