Hot take: Bruiser build is troll
As the title says, coming from a guy that LOVES bruiser builds, I think bruiser lee sin has been completely outclassed.
Specifically, I think going conqueror over electrocute is just mindlessly following an outdated build, the new meta is so late game focussed and damage heavy that realistically any advantage conqueror may have provided in the early skirmishes just completely disappears post 20 minutes (and closing games pre-20 minutes is just not realistic anymore) because you will just get oneshot before you can fully stack it.
Electrocute instead offers a big enough damage addition to REALLY make lee feel like the early game menace he should be, allowing him to finally deal with some very problematic matchups, and also allows you to access amazing runes that normally you wouldn't run with conqueror (sudden impact, ultimate hunter and especially the trinket haste rune which makes buying control wards almost optional)
Unless the enemy team is genuinely all tanks/bruisers there are virtually zero scenarios where you should go bruiser over lethality/full damage.
Bruiser lee sin is a jack of all trades master of none, but in the worst way: you don't have enough damage to one shot carries, you don't have enough durability to survive said carries, you don't have enough sustain to beat another bruiser (playing bruiser into ANY stat checker is MISERABLE), you scale terribly and if you're not 5 kills ahead you just don't get to play at all.
Full damage electrocute build fixes almost all of these problems and especially the last one: not only do you have enough damage to one shot carries, but due to how disgustingly high damage outputs are this season, you get to just beat the hell out of bruisers as well.
You also become more slippery and get to take full advantage of lee sin's mobility without running out of energy because instead of going constantly in and out, or having to stick to the target continuously to finish them off, you get to just delete them almost unreactably and then dip back to safety waiting for your Q to come back up and finish the fight with your team.
Going back to the comparison, personally I've always found that, bar some cases when lee was actually meta, the bruiser build struggled to really 1v9 unless genuinely like 10 kills ahead, but assassin lee just snowballs out of control and much more easily opens options for shutdowns when behind. He also scales incredibly well and is surprisingly easy to survive with compared to the past.
For reference, the build I'm using right now is Electrocute with triumph and alacrity - Bastion breaker, omnivamp boots, voltaic cyclosword, LDR and lastly optional defensive items like maw, DD, GA or occasionally a very helpful serpent's fang.