One (amongst many) NTP server going down creates less issues than an NTP server spreading wrong time.
General rule of thumb: a misbehaving/slow server in any well-architected distributed system is vastly worse than a dead server.
technically if you have 3 or more sources that would be caught; NTP protocol was designed for that eventuality
General rule of thumb: a misbehaving/slow server in any well-architected distributed system is vastly worse than a dead server.