Bad news: Many ECM server products claims CMIS compatibility, but in fact many do not implement the standard correctly.
Good news: There is an easy-to-use open source tool called "TCK" that checks compatibility of an ECM server product and produces an HTML report with metrics.
Bad news: Many ECM server products have no trial version, so only people who have bought it (usually extremely expensive) can run the TCK tool on it.
QUESTION: Is there a place that gathers the TCK results of each ECM server product?
Hopefully a place where anyone can contribute their own results, under an open license.