Apologies for the length of this post, but hopefully it helps someone. As @rlithgow1 notes, dependency on the cloud is a big issue with most wifi devices, along with the fact that most consumer routers are just not designed to support the number of devices that can be simultaneously active in an automated home. They might get an IP address, but the traffic can become an issue. But manufacturer cloud reliability is often the Achilles heel.
Here's my personal experience: Just prior to COVID, we had three properties in two states, all running some level of automation and one with two separate buildings that each had its own network. One property had a business class (commercial) cable internet through Comcast that was highly dependent on the cloud except for pre-programmed devices like thermostats. Another property had FIOS and only cloud-dependent devices, and a third had a "sketchy" internet connection via satellite with consumer-grade LTE backup (a rural property) but very little cloud dependency by design, because of the less-reliable internet at that location. As it turned out, the most reliable of those three properties in terms of staying "up and running" was actually the rural property, NOT because of the property's internet connection (which was by far the weakest), but rather because it had no almost no dependency on cloud-based devices. I don't think we ever lost the FIOS connection but manufacturer cloud services did go out, several times. The business class cable internet went out more than once, and similar dependencies on the manufacturer's cloud existed, so that property was "down" the most, even with a business-class internal backbone (router, switch, APs, etc.,) which never failed or even hiccuped. Again, the problem was the manufacturer's cloud services. But the rural property, with many automated devices, never truly went down because it wasn't cloud-dependent. Sometimes it was achingly slow to communicate when I logged in to check on it and it was limping by on the consumer-grade LTE backup connection (e.g., when snow covered the satellite dish), but the property itself just chugged along, communicating over its "internal", local network (z-wave, in that case). We did have one cloud-dependent thermostat that would run a pre-programmed schedule even when it lost its connection (a Nest, in the garage) and that was the device that was by far the least reliable. I replaced it on our next trip back to the property with a GoControl, which has been flawless.
We made the decision to move full-time to the rural property when COVID hit, and have decided to stay there permanently (just accelerated a retirement plan before the retirement itself). So, we upgraded the internet as much as possible to allow for work from that location (three simultaneous, redundant ISP feeds from three different suppliers, which makes internet cost more than all other utilities combined!) so now we stay up and running reliably in terms of the internet. But do we still see outages with a few devices? Only when the cloud provider is having a hiccup! So, we restrict dependency on cloud devices/services to "convenience" things such as Alexa. The cloud is a wonderful thing, but IOT/HA suppliers whose services depend on it still have outages more than many of us find acceptable. So, local control is what many HA enthusiasts still advocate.
Hope that experience helps someone going forward... Cloud-based devices and services are great, but only for convenience. When it MUST work, keep it local. And for truly essential things like heat, I'd have at least one backup system that reverts back to a truly "dumb" device, with as little automation as possible (e.g., a mechanical thermostat wired in as a failsafe).
