the downside is that the go runtime doesn't expect memory reads to page fault, so you may end up with stalls/latency/under-utilization if part of your dataset is paged out (like if you have a large cdb file w/ random access patterns). Using file IO, the Go runtime could be running a different goroutine if there is a disk read, but with mmap that thread is descheduled but holding an m & p. I'm also not sure if there would be increased stop the world pauses, or if the async preemption stuff would "just work".
Section 3.2 of this paper has more details: https://db.cs.cmu.edu/papers/2022/cidr2022-p13-crotty.pdf