It really depends on the sender. For instance, if it is a fatal event, it is unlikely that the sender will be able to resend an unacknowledged event. In general, informs are more reliable than traps, but there is no guarantee that a sender will resend an unacknowledged inform, only that the sender can resend an unacknowledged inform.
This is how Cisco explains informs, SNMP Inform Request:
SNMP Inform Request
The SNMP Inform Request feature supports sending inform requests. SNMP
asynchronous notifications are usually sent as SNMP traps.
Traps are less reliable than informs because an acknowledgment is not
sent from the receiving end when a trap is received; however, an SNMP
manager that receives an inform acknowledges the message with an SNMP
response PDU. If the sender does not receive a response for an inform,
the inform can be sent again.