They fully understand their limitations. Users of accessibility technology are extremely good at understanding the precise capabilities of the tools they use - which reminds me that screenreaders themselves are a great example of unreliable tools due to the shockingly bad web apps that exist today.
I've also discussed the analogy to service dogs with them, which they found very apt given how easily their assistive tool could be distracted by a nearby steak.
The one thing people who use assistive technology do not appreciate is being told that they shouldn't try a technology out themselves because it's unreliable and hence unsafe for them to use!