I personally have never worked in a team where Agile (the concept) has failed. But I've also seen it fail all around me. Especially when it's mandated without buy-in. Or when people just don't "get it".

e.g.

- 45 minute "standups" (!?)

- PI "planning" that consisting of deadlines and glorified multiplayer MS Paint

- Rigid adherence to ceremonies or processes that add zero value

- Retros that focus on complaints and venting with no actionable outcomes

- etc etc

Every time I've introduced Agile to a team or project that was new to it I was always met with skepticism. But 6 months down the line noone on the team/project wanted to go back to the "old" way of working. I don't even really care about any text book definitions. These are the only things we try to stick to:

- Short, daily standups

- Planning based on risk reduction

- Estimates based on complexity (ties in with risk reduction)

- Actionable retro items

- User demos every sprint (makes it easier to pivot - users rarely know what they want)