One major difference leap day is not inserted by adjusting clocks by 24*60*60 seconds into monotonic clocks. But leap seconds are handled this way in many time sources.

I think the concept itself is fine, but software developers screwed up implementing it when designing unix and NTP time, or how operating systems handle hardware clocks.

Now there are unix and NTP timestamps that don't refer to a unique time point due to leap seconds, as they were rewound by a second at the time point when the leap second occurred. Somehow nobody thought that it would be a good idea for unix and NTP times to be rewound by a whole day after a leap day occurs.