MIPS is mature, all the toolchains are here. It's simply that most vendors are too incompetent to care, as it doesn't affect their revenue in any way.
> Security and Hardening by Steven Barth - OpenWrt Summit
> Introducing new and ongoing security improvements to bleeding edge OpenWrt. This talk describes the new default toolchain featuring musl libc, enabled hardening including ASLR, SSP as well as OpenWrt's novel jailing (jailfs and seccomp) and package signing features (based on Curve25519).
> https://www.youtube.com/watch?v=DNYvIrI5Bbs
On the other hand, OpenWrt implemented full system-wide ASLR recently. NX and jailfs is still a WIP.