If Fletchling faces off against a torchic that has its berry juice intact and has full health, Torchic will both win and be able to successfully pass. If Torchic has taken either one substitute's worth or SR damage then Fletchling is only slightly more likely to win (it's actually almost a 50/50, funny how those things work) if Torchic is running the spread that lets it survive against fletchling.
196+ Atk Fletchling Acrobatics (110 BP) vs. 0 HP / 196 Def Torchic: 15-18 (71.4 - 85.7%) -- 56.3% chance to OHKO after Stealth Rock
In addition, if you try to predict a will-o-wisp and SD instead of attacking, you're actually worse off (Burn actually lowers your damage more than SD raises it due to some quirks in the formula).
+2 196+ Atk burned Fletchling Acrobatics (110 BP) vs. 0 HP / 196 Def Torchic: 13-16 (61.9 - 76.1%) -- 6.3% chance to OHKO after Stealth Rock
So yeah, at the very least a torchic vs fletchling lead ends pretty well for Torchic.
I have a few replays to help illustrate some of the difficulties Baton Pass faces, and its reliance on prediction/luck going its way.
Replay 1: Me vs Apt-Get
http://replay.pokemonshowdown.com/lc-218531957
Before the match even starts it's obvious that issues will be had in the form of a very nasty cottonee, but regardless of what it looks like, the match could actually have been less of a fletchling murdered everything. On Turn 5, where Apt made a spectacular double switch, if I had managed to predict that in some distant, bizarre corner of my mind and instead used barrier again, it would have become much more difficult for apt, but the game ended far too early for me to accurately judge what would have happened.
Replay 2: Me vs Teddytom
http://replay.pokemonshowdown.com/lc-218537908
Once again, my fairly meager prediction skills let me down as I fail to use will-o-wisp on his Corphish's Dragon Dance. Instead I get lucky and win the speed tie with my scarf Gothita and then proceed to end up sweeping, although Teddytom could have won if he had danced on Munna and relied on Aqua Jet in order to sweep.
In general the opponent always has at chance to win where I could do pretty much nothing about it. Baton Pass is so reliant on being able to read your opponent, and hoping that they don't quite make the right move to screw you over, and
even then relying on a crit not to happen to you right when it would screw you over that it's not even a very good strategy, forget broken. Against a good player it's much easier to spot weak points, while lower ladder players tend to be worse at seizing opportunities and applying pressure.
On the other hand, these replays should show that
if a nerf to FullPass is needed (big if), then BP would need to be reduced to 2 members per team. 3 members was sufficient in pretty much all cases where BP actually pulled it off.