As far as I know only a single new gTLD has "failed" (.wed) and even it is at moment still limping along in EBERO status (I think). It had a bizarre business model with $150 for year 1 and $30,000 for year 2 (really! Who thought that was smart?). It launched with a single registrar handling it, and I think only ever had 2 or 3 registrars, and only ever had a few hundred registrations. And even it is not yet non-operational.
It gest back to the whole idea of "experts" not understanding the business, Bob,
The whole NGT program originated because of the artificial demand caused by Domain Tasting/Kiting. Most of each day's drops in COM/NET/ORG were being hoovered up before ordinary domainers and users could get to them. Thus the whole idea that all the good domain names were taken started to propagate. There was also a dark side to this in that brand names such as Dell were being targeted by bad operators who would register tens of thousands of variations of the brand names in combination with whatever computer or offer was popular. So the ordinary would-be registrants couldn't get domain names and brand owners were getting very upset with ICANN and its failure to stop the abuse. So the new gTLDs (NGTs) were seen as the solution. So for $185K a pop, a company could, once it passed evaluation, get to run a gTLD. As with any new business, this did attract a lot of complete spoofers passing themselves off as experts and promising the gullible millions of registrations and untold riches. But ICANN was shamed into enforcing a kind of restocking fee for domain names deleted during the Add Grace Period. It had been free from registrars to register and delete domain names during this five day period. Domain Tasting almost collapsed overnight and the floodgates were opened. The artificial demand for many of the new gTLDs had disappeared. Legal action was also taken against some of the worst abusers of brand/trademarks and these registrars were shut down.
The weird thing about the .WED business model is that it makes sense in a non-domain name industry sense. There would, in this theory, be a finite number of combinations of names and they would be to advertise a one-off event. The domain names wouldn't be needed after the event. The problem is that the domain name business doesn't work that way. TLDs make most of their money from renewals. The new registrations matter but the renewals are solid, repeating income.
Some of the registries completely underestimated the costs of marketing and the demand for their gTLDs. They were frequently single gTLD operators. Some of them sold out to the portfolio operators like Donuts. These gTLDs were not successful in their original incarnation but the transition from single gTLD registry to being part of a portfolio operator generally went unnoticed by the end users. Since the deals are done without the NGT entering EBERO, they don't appear as failed gTLDs.
While it is true that the extensions with the most solid backing and sound business plans are most secure, but so far, and we are now 5 years into it, despite the struggles the number of new extensions which are open to anyone to register that have failed is I think 0 (but the $30,000 per year 2+ .wed is on life support).
The .WED has been frozen since January 2018 and it is even eternally pending on the CZDS. The .WEDDING is currently around 22K registrations. (The single/plural/variation thing is a whole new set of arguments. The backing and business plan matter but the fate of TLDs is decided by demand. If the registries don't promote their gTLDs and don't help registrars to promote them, then the gTLDs just don't get traction in the market. The blood trails on gTLDs that are in trouble are visible in the registrations, renewals and usage. By usage I mean real web usage analysis rather than the cargo-cult rubbish. It is possible to estimate which domain names will not renew based on a combination of these statistics and an algorithm. The correlation between poor web usage and development and low renewal rates is quite stark. One NGT had a renewal rate of 0.58% for 2017 registrations and the 2018 renewals are going to be as bad. (The multi-year registrations are also an indication of how a gTLD is perceived by its registrants. More multi-year registrations and renewals are good.) However, the removal of these low quality and heavily discounted registrations from the NGT is a good thing as it will, theoretically, allow the NGT to recover. It can take about five years for newly launched TLDs to stabilise. The completely screwed up launch pattern (another ICANN mess) means that some NGTs are only on their first renewal while others are on their fourth. Thus the XYZ and TOP have millions of registrations and .APP is approaching its first renewal and .DEV is still, technically, in its land rush period of operation.
Regards...jmcc