How do mini penny-farthings ride compared to regular size pfs?
My impression is they're a toy only, not to be used for travelling, just tootling around a park or around home.

My unicycling coworker has said that they are essentially unicycles with a training wheel, and are a lot of fun for playing about.  Also, I've seen video of them being used in street performances like a juggler or similar.

But no, you wouldn't commute on one, and the ergonomics would make training on one uncomfortable.   The 29" wheel would suit a kid, not a grown adult.

Additionally, with no gearing, their top speed is limited  to one wheel circumference per pedal revolution.  So this 29" wheel will have a rolling circumference of about 29" x pi or about 91" or 2.3 metres.  At 60 and 80 RPM that would be 8.2 km/h (5 mph) or 11 km/h (6.8 mph) which is a fast-walk/slow-jog to an "average runner's speed".

Said another way, they probably fixed gears so you cannot coast, you have to pedal.

The rear brake will be mostly useless - the common wisdom is that the front brake of a bike does most of the real work of braking.  Trying to stop with the rear brake only simply drags it out longer.  Your mini-farthing will likely stop poorly on brakes, relying more on fixie-skills like reverse pedal pressure, and that its overall speed is low.

I'm sure they're a lot of fun, but not a lot more than that.  With practice and good technique, I'm sure they can be ridden all sorts of places, but perhaps not aggressive/risky offroad/downhill etc.

  Example of a MPF with quite a lot of rake.

 Highly vertical forks with minimal rake.

The two bikes above would have massively different handling - no conventional bike would risk having such a vertical head tube.