It being google, if you aren't buying brand new, your security updates are of a (too-)limited duration. You have to judge how important that is to you.
I have a X4 and its a fine device. Probably more than I need. I was on a Nexus 5x which was great until I saw three or more friends do the boot-loop-of-death. They had a trade-in special on them when the X4 came out and, having seen mine beginning to show the first signs of the that problem (very occasional random reboots, which it hadn't done during its 1st year of life), I happily made it "someone else's problem."