I don't know about you guys, but I am starting to notice some of the LED streetlights that my city has installed are already failing; and I've even seen a few be replaced back to high pressure sodium. Has anyone else noticed them failing early? (Note: this isn't an anti-LED post, I'm just curious)

1. Learning curve on how to properly design with a new technology - still a lot of design errors (e.g. it is still not clear even today, how to handle the wear of critical components and their maintenance - no standards,..; compare with a E40 socket and fixed lamp specs for HIDs)...
2. New, so expensive technology, so more aggressive cost cutting (operating the LEDs at or close to their design limit to save the needed amount of them).

When these two combined, it becomes a deadly coctail for the lifetime.

And because the nature of the LED (a lot of internal connections prone to failure,...) the MTBF is what dictates the lifetime andnot the predictable wear.
Plus on many installations the end of the designed life is here (24kHour means about 5 years).
And many installations are brand new, so early failures are frequent.

I would expect the same evolution as with any new technology in the past. Only for such infant technology they are used way too widely, so the impacts of the failures are more visible.

The major issue we have here is not with the LEDs, but the power feeding the pole.  all wires are underground here so we see a lot of blown pole fuses and wire faults.  Non-dimmable drivers don't like power reductions. 

In 2016, about a 4 mile section of the highway here was replaced with LED fixtures. I have noticed, that sections of them do not work and haven't worked for months now. I think it has something to do with the power supply itself than the fixtures. It is surprising they haven't bothered to fix them in that long time since they are LED.

The HPS fixtures on the other hand mostly all appear to work, with the exception of a small handful, maybe two or three out in a 6 mile range.

I hadn't noticed any of the LED Evluma fixtures in my neighborhood not working.