Don't even question people's builds, period.
Generally speaking, you just look like a dickhead. Best-case scenario is that you can claim that it's bad based off the 1-game sample pool that you've encountered it. Worst-case scenario the Lethality Lucian you've been shittalking for 20 minutes because "you're meant to build crit noob" instagibs you with his ultimate from complete safety.
Unless you play with the person building weird stuff regularly and it never works, don't even question the build. I've seen some pretty weird builds do ridiculous things, and I've built some weird things that worked for me for entire seasons (Guinsoo's + Gunblade + Abyssal into Tank Jungle Fizz was pretty good after its first rework at the start of last season). Hell, half the time the 'weird' build you're talking shit about was meta at some point and never actually got any worse (Magic Pen Tank builds are still incredibly strong even though that was more of a Season 3/4 meta).
against an all AP composition. My runes and masteries were set for being able to flex either an AD or AP as I wouldn't know my opponents until the load screen. I opted to run AP as there was a lot of poke and burst invested into the opposing team. Running full MR tank would only soak but so much damage from the loaded AP and M.Pen built. And despite them being glass cannons as well running AD would have been an option. But that would have mitigated my E once I got close. I would probably get one W-Q and some of E in a AD combo and probably deal less than a connected R-W-Lich Auto in a AP combo. Most Ults were pretty much forced initiation or disengage as a result as well.