Part of my job is working on a in-kernel key/value store for a data center network operating system. The problem it is the context switching between user and kernel space kills your performance if you're targeting sub-millisecond read/writes. That may not matter for something like acunu when you factor in network latency but when you're using the database as part of the packet path it does. In addition, under a high system load your user space process has a high likelihood of getting scheduled out at the ioctl call which makes latency even worse. Although being in the kernel allows you a little leeway in terms durability constraints and all that because if you screw up the entire system comes crashing down anyway.
It will never be in the mainline kernel. Also, although I haven't actually looked at what they did yet, I assume they're just loading a regular old kernel module instead of actually really messing with a lot of the mainline code.