Have you seen an instance where the stsadm preupgradecheck passed on Features, but the Test-SPContentDatabase found errors and the mount failed?
There were multiple versions of the pre-upgrade checker. It first arrived in SP2, but was updated with the Oct 2009 updates.
Overall though I've always found that the Test-SPContentDatabase is the most accurate, and if it identifies problems then you will have issues completing the upgrade. Resolve those issues and then proceed with mounting the content database.