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)
If you don't care about definitions, then it would be good to not perpetuate using the word agile for your own set process. "Individuals and interactions over processes and tools"
Venting is important. When you don't, tension builds and then explodes. It's necessary to give people a way to air complaints and be heard. And if your team has people with some organizational and social skills, you can channel that into action.