My experience with solid-state batteries differs significantly from yours. Maybe it's just colder here in North Dakota. :)
Also, the Prius's heater uses engine coolant. It apparently has an auxiliary electric heat source and water pump to keep the cabin heated when the gasoline engine is shut off, but if they put the car together in a remotely sane manner that should result in redundancy rather than an additional point of failure. If the batteries or the gas engine fails you in the Prius, you still should not freeze to death.
In a Tesla, though, if the batteries fail you then you will freeze to death. While the same is true for the fuel line and pump on a gasoline car or, much greater risk because of fuel gelling, on a diesel car, I have not personally witnessed a properly maintained gasoline car that started and reached operating temperature fail because of fuel delivery issues, in a climate where -40C happens at least annually and for roughly three consecutive months each year the temperature does not exceed -10C on more than a small handful of days.
And that's what I mean about the Tesla being unproven. We have a century of gasoline and diesel engine experience so we know how to keep them working in extreme cold and the risk of problems is a known quantity. We also have a solid understanding of how to survive in the event of various problems that can arise. (As an example, if you are stuck in the snow but the engine is running, you ensure that the exhaust is clear so it can escape to atmosphere and then you stay in the car for warmth and safety.) For a hybrid like the Prius, the risks should be both similar to those in a gas car and also mitigated by intelligent engineering of redundant systems. They are still uncommon here, but that is likely more because they have low ground clearance than because they are hybrid. For a purely electric car, though, we have no experience keeping them running in extreme cold and the risk of problems is an unknown quantity. We also do not have a good understanding of how to survive if a problem arises. What do you do if you are stranded in cold weather by a dead battery and help will not arrive until morning?
Everything I have identified as a risk of electric cars can surely be quantified and likely be mitigated. But with gas cars we have a century of experience to draw on. It will be a long time before the same can be said for electric cars. And during that time, I will let people in populated areas where help or alternative transportation is within a 5-minute walk at all times learn and fix the problems before I incur the risk of a frozen battery leading to my own freezing to death.