(assuming we're ignoring boots in item counts), a post BF sword ADC is about the same as everybody else in terms of power. But a zeal + BF adc is far weaker than almost any other item and a half build, and an IE + Zeal is finally something, but still feels a little worse than 2 full tank items (sunfire/randuin) or 2 AP items (Morello/ludens). 3 Items feels even slower because you're not hitting harder, just faster. Now compare that to a tank top of Sunfire/Randuins/SV or something like that, or an AP mid of Morello/ludens/Rabadons. At 4 items (presumably double zeal + IE and BT) you finally feel like you're hitting hard against squishies, except by now you also need a LW to deal with tanks. So at 4 and a half items, you're finally able to do buckets in teamfights, except at this point the AP mid has Morello/ludens/void/rabadons and possibly zhonyas, the toplane tank has Sunfires, Randuins, Spirit Visage, and some mix of GA, IBG, ZZ'rot, Warmogs, Locket, Knights Vow, etc.
This is the biggest reason why ability dependent ADC's (vayne RQ crits, Cait headshots, Twitch stealth ults) are some of the stronger ones right now, as opposed to purely auto-attacking ones (sivir, ashe, Trist). Those top 3 have abilities which give artificially high AD, making up for what they lack as part of their build, through Vayne and Twitch ults, cait's traps and headshots, and they all get false "burst" from those interactions with crit.