What I've seen is they require fancy materials (carbon fiber, etc) due to the extremely high rotation rates (>100k rpm). For maximum efficiency, the flywheel needs to be in a vacuum to eliminate frictional air resistance losses. They also need magnetic bearings.

None of these are exactly a problem, they just cost money to implement properly.

I would say this copy + pasted list is not great.

No HVAC required - ok but you do need to suck down a huge vacuum, which is a lot more difficult than traditional HVAC

Hot and cold - see above point

All steel - not sure about that, maybe for lower speed applications, which reduces the amount of energy stored

Safe & reliable - hard to say that a huge mass spinning >100k rpm is "safe". By itself there is no fire hazard sure. but what if your magnetic bearings fail and it hops off and starts trying to escape it's (vacuum) enclosure. Then you have tons of sparks and pressure differences and other non-fun things.

O&M - [citation required]

30 year life - surely with all the plusses listed, it should be longer than 30 year life?

I think rather than think about it as "pros and cons", a better approach is to think about where these various types of storage are used. I think the analogy of hard disk drive -> RAM -> on-chip cache for memory storage (that is cheapest/longest lived/slowest to most expensive/short term/fastest) also works well for energy storage:

1. Pumped hydro is currently best for long term-storage. The lack of other cost-competitive long-term storage options is currently one of the biggest issues with renewables and an active area of investigation.

2. Batteries are good for daily smoothing (i.e. when the sun goes down), but they have limited charge/discharge cycles, they lose charge over time, and they don't, as of yet, have storage capacities needed for long term storage.

3. As the article points out, flywheels are good at instantly changing output based on changes in the grid, so they're good for frequency regulation and onboarding/offboarding new energy sources. Their biggest downside is their limited capacity compared to other tech.

Wrecks entire village in case of catastrophic failure