An ultimatum speaks of consequences if unheeded. A necessary condition speaks of whether or not progress will be taking place, which may or may not have consequences.
Whether these are "good" or "bad" things is entirely subjective, and has no meaning without any context.
If you ever give an ultimatum you have to be willing to follow through or it means nothing. Conditions are open to negotiation.
Neither are bad. Whatever you need for you will be right for you.
An ultimatum is an absolute wheres a necessary condition could conceivably leave room for negotiation. Neither are bad if you wish to maintain certain standards.
The way you present it. Hey are neither good nor bad.
Ultimatum is making someone choose you or something else usually in a relationship I hear this! And necessary condition is just that a condition!