He is really good as 4 star
As 3 star he is still decent
Since his passive is not a buff and works all time like AGK or Madara Konoha Founder you c*e him with 5 kage Madara
If you want to use 6 paths Naruto and Azure Fang you c*e them + 5k madara +chiriku, will be a fine team
But I would recommend to get him 4 star somehow

He is good , is even better at 4 stars , but also great as 3 star ninja , has that buff that increases res to konoha ninjas , it will be very helpful with ninjas that are not that tanky such as join minato and shisui , also it can be used with 6 paths as well , ashura naruto is already tanky as hell , imagine buffing him with that res buff + his healing , he will be invincible

He is good if used together with hokage minato or with triple healing team. But he is relatively squishy.

I am not even sure if his buff works, just like Edo Hanzo when he was in 3v3 arena, he was bugged and broken af, and as far as I am using him with multiple mains, I haven't noticed any 40% increase in my def and res, I can't really tell if it works at all, I mean...c'mon...40%, it's a lot, but I feel like it's not even there, all ninjas take damage like before.

This.

Also, idk if his buff stack with earth's passive. They have different targets but same increase so I'm not sure.

In theory, if I understand correctly, everything stacks with Main ninja buffs, no matter if equal %, but it doesn't stack if the same ninja gives same field buff, or different ninja gives same stat % increase. I tried with Earth Main too, and I still wasn't sure if it worked, but it *s and it's not 40%, or it doesn't work at all.

Chiriku is definitely a great ninja to use when you create a lineup with other Konoha ninjas.

Also, regarding if his buff works or not. Chiriku's buff does work, you can find more detailed information here courtesy of ICE*.

Make an ultimate Kushina team (U need full skillbroken shikamaru a 1 Kushina)

40% def+res from Shikamaru, 40% def+res from Chiriku, and if possible in future, get yourself Kushina passive skillbreak which gives her Res+Def everytime sb hit her

Shikamaru gives 100% def and res. Gotta try this team it is a new sort of cancer xD

His worse version of passive give 100% def/res but only for 2 rounds, his Mutation version of passive gives 40% def/res till he is on field.