There's nothing at all suspicious about the 172.16.0.0/12 address block.

Correct. However the 172.16.42.0/24 block is the default for a WiFi Pineapple. Any other range is usually ok, but the 42 on a /24 specifically would make me (and most people who actually know what a WiFi Pineapple is) avoid that network.

It's just one of many private ranges.

Sure, it's one of many, but how often do you see that specific (42) block used? I honestly don't think I ever have, outside of a pentest lab. The 172.16.42.0/24 (not just any 172.16 like you're saying) block is the default for a WiFi Pineapple. Any other range is usually ok, but the 42 on a /24 granting WAN access specifically would make me (and most people who actually know what a WiFi Pineapple is) avoid that network.

Saying 172.16.0.0/12 is usually for pentesting scummy thing is very misleading...Saying it's dangerous is like saying every websites using .xyz domain is dangerous(which makes little more sense than this, btw)

You clearly don't know what a WiFi Pineapple is, because we're zeroing in on the 42 and you liken it to the entire 172.16 block. And linking every .xyz domain to a specific block (42) that is used by default for a pentest device is even more misleading.