It looks like there won't be a migration path, and you'll need to rewrite your app from scratch using a different language. Again, it would be nice some transparency around how much VC money they've taken.
I think the only way I'd use this is if I had the option to host it myself i.e. the code was all open sauce.
For our early customers, we have a handshake agreement that we won't leave them hung out to dry. If Dark was to shut down, we would 100% make sure they had the time and ability to port their app off. There would be a time cost to them of course, so it's not perfect.
As we get closer to opening up to the public, we'll be doing a bunch of work on the sustainability question so we have good answers to the "acquired or shutdown" problem. We imagine a legal framework to protect customers, but haven't got the details worked out.