For me, it's the fact that we aren't (currently at least) using rust. I would possibly, maybe, consider porting it to another language but haven't had the time...
In general I wonder how many people sit in the intersection of those who are free and willing to base their system on rust, have dataflow problems to solve, and understand the advantages differential dataflow brings to the table?
Do you think you would be using it if it had bindings to $WORK_LANGUAGE?
Figuring out how to manage memory without constant de/serialization would be tricky, and it's unfortunate that it's so hard to do fusion across FFI, but I'd still expect it to happily outperform eg spark sql.
In general I wonder how many people sit in the intersection of those who are free and willing to base their system on rust, have dataflow problems to solve, and understand the advantages differential dataflow brings to the table?