Repeating something I posted in another article:
I played around with some random numbers in the
page Zeiss has set up for determining if your prescription is supported.
It rejects anything with prism outright, but for more common SPH and CYL corrective lenses, it looks like they'll accept up to -10.25 SPH or -4.5 CYL alone, and there's some equation that limits how much of the two can be combined (I assume due to how much glass is necessary to provide the correction).
For example, if you had -10.0 SPH you can't get more than -1.5 CYL, but if you have -5.0 SPH you can get up to -3.75 CYL.
All of which to say that they will make inserts for moderately severe myopia and/or astigmatism, but there is definitely an upper limit above which you're out of luck, and if you have both, the limits of how much can be corrected are narrower.
It occurs to me that an annoying side effect of the US treating corrective lenses as medical prescriptions is that, if you were just above the limit, you can't just say "Eh, it's a little soft, but I can get by fine" and get something close to what you need. I can get by without prism without double vision for a while, my eyes just get tired, but I can't make the call that I'm okay with that because it's not what my prescription says, so they won't sell it to me.