I remember using darcs, but the repos I was using it with were so small as to performance really not mattering…

I remember darcs fondly but even with tiny repos (maybe 5-6 people working on it) we hit the "exponential merge" issues.

It worked just fine 99% of the time and then 1% it became completely unusable.

I definitely miss Darcs. I still use it very occasionally, but only with very small repos.