Baton Pass teama weren't broken in Gen II
Baton pass teams were powerful in Gen III and that was resolved by removing Ingrain from Smeargle.
Baton pass teams weren't broken in Gen IV.
Baton Pass teams weren't broken in Gen V.
Why Baton Pass teams are potentially broken in Gen VI?
Only in Gen III Baton pass teams ere considered uncompetitive but with a key differences: the metagame of Gen III has heavily stall oriented, unlike the one ater it. Also in Gen III:
-There were no physical/special split yet. Meaning that many pokemon can't utiize some of the STABs because they don't have the stat to use it, making them unviable.
-There were almost no priority moves outside Quick Attack and the rare Mach Punch.
-A lot of powerful moves didn't exist until after:
Fire doesn't have much abuse in physically based pokemon forcing to run a mixed set, which hinders them more than in Gen II by how EV works in gen III.
-Water doesn't have physical seppers with Waterfall and Aqua Tail.
-Grass: Giga Drain was 60 BP, Leaf Blade and Petal Dance were 70 BP. Also, moves like Energy Ball, Seed Bomb, Power Whip, Wood Hammer, horn leech and leaf Sotrm diudn't exist.
-Close Combat didn't exist in Gen III, alongside other fighting moves.
-Except 3 pokemon with Drill Peck the best Flying STAB was Aerial Ace or HP Flying. No air Slash and Brave Bird.
-No high powered Bug mvoes outside Megahorn Heracross. No other Megahorn users, no X-Scissor,,no Bug Buzz, no the annoying U-turn.Silver wind has weak and physical and Signal Bean was only on Volbeat and Dewgong.
-No Stone Edge or Head Smash to attack.
-Gengar can't fully exploit Shadow Ball.
-No Draco Meteor, no physical Outrage (and was 90 BP), physical Dragon can abuse DD + Dragon attack.
-Apart of Dark missing a lot of current moves like Night Slash, Sucker Punch, etc. lack pf special busers of Dark moves.
-No entry hazards, like SR, Toxic Spikes, etc.
And more things.
Meaning that the metagame of Gen VI can deal with sheer power better than the Gen III metagame.
what is new to Gen VI that makes BP teams "potentially broken"?
-Fairy types
-Scolipede getting Speed Boost.
Mr. Mime is still Mr. Mime even with Fairy type added in, which is no way uncompetitive. Sylveon is a seriuous threat, but nothing from ordinary and can be dealt with standard methods (ecept Dragon Tail for obvious reasons), Mawile is not that big of threats with his low Stats, Mega Mawile is very slow (even at +1 is slow) andlost potental power by making in a BP teams meaning "I need boosts".
That is the thing that stands out? Scolipede getting Speed oosts makes:
-Can Protect in the first to get an speded boost and guaramnteed to go first.
-If a special sweeper comes, can hide in a Substitute, (always going first now), needing priority to defeat it and getting a secinds boost that makes uninvesteed threats like Sylveon "fast" when BP.
-If a physically sweeper appears, can use first Iron Defense to boost his defenses and passing them to his teammates who are more specially bulky in general.
wirthout it, there were no mebers who can pass Speed and Deenses at once, speed has to be passed manually (getting hit in the process), making the team more difficult to pass up.
The objective is not making BP unviable. A good BP teams shoudl be capable of defeat standard thrests regularly. The obejective is identify potential elements of BP that are broken and delete them.