From my personal experience: experienced teams can thrive with almost no methodology and an ad-hoc process because... They had experience with other processes and can see the good and bad in them.
I still advocate agile for less homogeneous teams or in situations like other posts have highlighted but a team of more senior developers with a working process that is open to be improved (one of the cornerstones of agile) will thrive with less churn than when forced into a by-the-book agile process.
For me Agile is by definition an ad-hoc process just one with guiding principles for how to go about organising it. The problem comes with formalised methodologies based on Agile which are treated as a one size fits all approach for any team.
I still advocate agile for less homogeneous teams or in situations like other posts have highlighted but a team of more senior developers with a working process that is open to be improved (one of the cornerstones of agile) will thrive with less churn than when forced into a by-the-book agile process.