I am assuming you are talking about WPA3-Personal (WPA-SAE).
The good things:
- adversaries should not be able to passively decrypt (present or past) communications even if they know the shared secret (which is an improvement over WPA-PSK);
- adversaries who do not know the shared secret should not be able to impersonate the access point.
The bad things:
- an adversary which has the shared secret can impersonate the access point (because after all it has the same information as the access point) i.e. by actively attacking the network it can decrypt/hijack the Wifi communications.
This problem can be solved by using WPA SAE-PK which associates a keypair to the access point. When using this scheme, the attacker should not be able to impersonate the access point unless it knows the private key of the access point as well.
The specific use case would be a public WiFi using WPA3, but with a common password posted for everyone (including the hacker) to know.
As explained above, this is not safe because the attacker can impersonate the access point. You could use WPA SAE-PK to solve this problem: in this case, you should be safe against cryptographic attacks (if all stations actually use WPA3-SAE-PK) however…
but who might also be connected to the same WiFi
As soon as the attacker can join the network, it might be able to trigger layer-2 attacks. You might want to check if your access point has protection against layer-2 attacks.