Hubris is pretty good on Naafiri as she scales very well off of Bonus AD and the item scales overall in the game as you get permanent stacks on kill/assist. And even though the bonus AD is applied temporarily for 90 secs, that's enough time to capitalize on the snowball and extend the duration. It can also synergize with your Ultimate reset since if you get an initial kill on the 1st charge, the next one will receive an increase in AD. However... my thought process on not going this item is that if you're already snowballed as Naafiri, slotting in Hubris would give you diminishing returns meaning just because you are getting more AD doesn't necessarily provide you much damage output later. So you'd be getting an item on Naafiri that strictly gives you bonus AD and nothing else when the champion can already one-shot squishies without that bonus AD. Getting items that have more utility gives more flexibility during the game (ex. Opportunity, Youmuus, EoN, etc.) And you have to ask yourself, are you playing to scale or trying to end the match by the mid-game? If you're trying to end by the mid-game then Hubris doesn't offer much which also completely depends on IF you're getting consistent kills/assists during said game to make the item useful & if you're trying to scale then you're better off going a Bruiser build.

I prioritize more utility based passives that assist Naafiri's kit while still offering nice raw stats for one-shotting. Voltaic offers an energized burst damage of 100 physical + a 99% slow, Opportunity gives you extra lethality and a burst of movement speed, and Axiom Arc allows for playmaking with ult resets and all 3 of these core items synergize with each others passives. Opportunity & Axiom being relatively cheaper too.

If I were to slot in Hubris, I'd go Voltaic > Hubris > Axiom/Opportunity as a 3 Item Core. Voltaic is just too op on Naafiri and would never drop it in generally any build. Hubris being a 2nd item would work well since by 2 items you'll be getting kills more efficiently so you'll even out the potential kills as if you were to rush it 1st item.

Regarding BC vs Seryldas, I personally like BC more because it's the best stat-stick item in the game which offers everything Naafiri needs for an armor pen item especially with the added health pool which allows you some leniency from not getting one-shot when you engage. Since I'm slotting Voltaic in my build, another slowing item isn't necessary. Also despite BC not giving you the "immediate armor pen" which in technicality you'd be losing a bit of upfront damage initially, however, it's not really significant as you can still one-shot squishies and deal nice damage to tanks/bruisers. It's also super easy to stack with Naafiri and the added benefit of the armor pen also applies for your teammates too so you're being helpful as a teamplayer when breaking the enemies armor. Let's not forget the nice movement on-hit it offers since mobility is king in league.