It dosen't work too well on some of the most infamous 1 shot monsters.
Most invasion bosses are powerful, but only a few has the ability to 1hko players and most of the 1hko invasion bosses do not currently work with the chicken on sight. Specifically tested are:
M'gska, the Living Pyre -> this guy's flameblast can 1 shot even at 75% fire resist.
Tested today, did not chicken. Definitely not working
Jikeiji - If all totem stack up, it gets scary
Tested 2 days ago, successfully chickened
Shivershell - works fine
Granitecrush - works, most of the time. Sometimes still engages him
Ossecati, Boneshaper - Vaal smash one shots. Tested Yesterday, works.
So the chicken script is currently somewhat unreliable, it works sometimes, and don't work some other times, and I'm really not sure what's wrong with it, but the above boss definitely need to be taken into consideration because they are truly "1 shot bosses" while others can largely be mitigated through resistance (e.g. chaos resist too) or do "chip dmg" that chips away at your health without 1 shotting most of the time.
will need to test a couple more scary ones