Aside from the obvious issue with engineering, physics, etc.

A unix second is allowed to be different from a "real" elapsed second.

The unix day has 86400 "unix seconds" which are actually 86401 real seconds (for days with added leap seconds).

Any application logging real world events against unix time will screw up velocities, force, energy, etc when computing "instrument reading" per "unix second".

It might not matter to most people but it's an issue for surveyors, geophysicists, astrophysicists, engineers, etc.

> Any application logging real world events against unix time will screw up velocities, force, energy, etc when computing "instrument reading" per "unix second".

That is a fair point for situations that depend on short term observations. But, UTC has the same issues there as Unix epoch. I think it is a valid edge case but if you are doing, say, GPS based speed calc I would wager you are already pulling your time reference from a low level source that you can depend on will be running steadily for the X minutes/hours you need it.

Sure, this is what responsible STEM people do for any time lapsed measurements, be those position, nine axis magnetic field recordings, gravity, radiometrics, microwave return, etc - they use an independant epoch based clock source that counts true lapsed time rather than conventional human time.

Point being, it's an area of UnixTime that many overlook - to date, since the 1980s I've had long term multichannel data aquisition running throughout every leap second adjustment which would have had data glitches had the time channel been UnixTime, UTC, etc.