The industry is coalescing around Kali as a security/pentesting distro, the way the did around Red Hat as the first linux distro to get certification with. It's probably great for free/low-cost training resources. Although apparently there's some Kali Linux books that aren't about pentesting at all, but just getting around Kali as a linux distro. Haha! Lesson One: hacking is mainly about social engineering, not technical knowledge, and that includes "white hats" selling themselves and their products. Secondly, as far as pen distros, the Katoolin script is pretty good. I haven't been able to use it much, but installing it is a blast, and you can put it on any Ubuntu based distro. I haven't tried it on a Raspberry Pi- that could make for some good set-ups. Likewise you can also use the "Blackman" script on any Arch distro (pacman script for Blackarch tools. Get it? Blackman?) Putting a good Archlinux distro together impresses me more than any custom Kali. Maybe you don't have anyone to impress with you're linux admin knowledge, but if you really want to learn... You could also try to update Pentoo or other Gentoo distro, but that might be much more work than is worth your time. Finally, I hope you don't touch Micro$oft's linux subsystem version of Kali, WSL or whatever it's called. That would be strictly for getting a certification out of the way. I'm really shocked they offer it, although it makes some sense in a sick, twisted sort of way. Script kiddies are customers worth tracking, too, I guess.