Yeah I've seen far, far too many LED failures occurring for me to even think about LED's standing a chance against MV regarding life expectancy. This includes
street lights. MV and HPS are both my personal favorites, especially since they last longer than the Metal Halide Cobras.
As much as I like Metal Halide Cobras, they are absolutely abysmal in terms of life expectancy. Metal Halides do not last long in Cobras, and I've seen recent
proof of this with the ones we have here.

Makes sense, standard “universal burn” metal halides don’t last very long at a horizontal position, any horizontal mount fixture, such as a cobra head, wall pack, or a shoebox, will cut the MH lamp’s lifespan from 10,000 hours down to 6,000. Nearly half the life. Mercury and HPS lights don’t care about the orientation.

They only achieve the 10,000 hours if used vertically (as in a post top, recessed can, high bay, or low bay). Metal halide lamps in general don’t last very long if they’re burned horizontal.

I get why MH wasn’t widely used on streets. I can’t imagine being an electrical technician always replacing the MH bulbs in high mast fixtures even if the fixtures can be lowered.

The only superior thing to Metal halide compared to the sodium was the color and CRI, which is why I even prefer MH. Everything else about HPS was better. The only benefit of MH compared to MV was better color and light quality, as well as better efficiency and CRI. Everything else about MV was better (no explosions, less premature failures).

Newer MH bulbs are also all junk, so no wonder they’re failing. Modern metal halide bulbs are known for being junk. Even reputable brands are now garbage products. The big name brands don’t make HID bulbs anymore. For a while Philips probe start metal halide bulbs were just rebranded Venture Lighting products.

PSMH was a bit better in terms of longevity and efficiency compared to pulse start, but they didn’t last as long as HPS and they tended to explode at EOL. Ceramic metal halide was expensive and not as long lasting as HPS. CMH would be expensive to maintain. That’s why Quartz metal halide was used instead. They were cheaper to run. Chicago, IL, who experimented with CMH cobra heads, experienced MANY premature failures with them. The electronic ballasts in the fixtures used failed frequently.

Universal burning 175W and 250W QMH have a rated life of 7,500 hrs when run horizontally (10,000 hrs vertically). Yep, they're a low performer, but the Philips Allstart CMH retrofit lamps for those systems have a rated life of 20,000 hrs plus, and they don't have near the depreciation issues. Same with 70W/C98, 100W/C90 and 150W/C102 CMH lamps.
Around four years ago, I installed several modified ITT Series 13's at a relative's house using 70W and 150W CMH lamps. The lamps are still going strong, with virtually no depreciation and the arc-tubes are still white.

Depends on the brand. GE ones have a rated life of 6,000 if burned horizontally (I often use GE bulbs as examples), I believe Sylvanias are 7,500. Not sure why there’s a difference.

I know this already has a lot of answers but I want to share my experience and opinions as someone who spent some time working on street lights for a utility contractor.

We had a decent amount of MH fixtures in the system. Most of them were decorative, like post tops and teardrops but there was the occasional cobrahead, flood, or shoebox. There are a few reasons that they aren't great.

As others have mentioned, they don't last very long. At one point we were told to just replace them with mercury vapor lamps and bypass the igniters if it was a PSMH luminarie. Some of these newer lamps would last less than a year and we would be repeatedly replacing them. The older ones we had in stock, you would get a few years out of but that's not a lot in a untility setting.  Usually the decorative fixtures and poles were owned by a municipality, the wiring, photocells/shorting caps and lamps by the utility, and we would service them for the utility. With that being said there was a lot of pressure from the municipalities for them to just work instead of constantly going out, hence the use of mercury vapor lamps.

Not only would they burn out really quickly, but the lumen depreciation and color shifting also happened very fast. Even when a string of lights was group relamped, within a few months they would start to shift at different rates and look pretty bad.

They're also very sensitive. I know that's not very descriptive, but that's because they're sensitive to everything, especially with age. Vibrations, it's too cold outside, the capacitor degraded a little bit, whatever. Situations where yes, it is an external issue, but a mercury lamp would still work.

Another thing is unlike HPS and MV lamps, there's way more versions of lamp under the same wattage. Universal burn, horizontal, vertical, enosed, open, psmh, probe mh, cmh. This caused a lot of confusion when they would order stuff. So of course we ended up with the cheapest option. Enclosed, vertical burn. You can imagine how something like this might last in a cobrahead or floodlight burning horizontally.

There were some exceptions where we really didn't have any choice but to use MH lamps. Some fixtures only used 400w cram lamps. We really didn't have much of a choice there. At one point we ran out of 175w MV lamps. We ran MV fixtures till we couldn't anymore, which meant that a lot of the newer ones in our system were able to run MH lamps, as they had Dual H39/M57 ballasts So a lot of 175w MV fixtures got MH lamps.

There were outliers, sometimes you'd replace an MH lamp that was 15 years old, but those were always older lamps, by the time I was working there, we only had ones that were of poor quality. There was also where I ran across a 400 watt HPS fixture with a bad igniter. They wanted it fixed asap so I temporarily stuck a 400 W MH lamp in it, until I could come back with a replacement fixture. They saw it worked, and said not to worry about it. It's been 4 years and it still works fine. Chinese made Philips M59 lamp.

At the end of the day, MH is more efficient, but they'd degrade so quickly that MV made more sense in the long run when you factor in labor, appearance, longenvity, and ease of keeping the correct lamps stocked. As better MH technology came along, so did LEDs, and here we are now.