A day doesn't go by that someone isn't asking a question in Slack #migration about how to troubleshoot a specific problem with a tricky migration. Almost universally, these problems be demystified by using Xdebug and putting breakpoints in two spots in Core's MigrateExecutable. First is in the ::import() method where it rewinds the source and then processes it. The second place I regularly put a breakpoint is in ::processRow(). Or if I already know which process plugin is breaking, I might put a breakpoint in it directly. For example, the sub_process or migration_lookup process plugins tend to be complicated and a common place for me to drop a breakpoint.

Try it out. Put some breakpoints in these places. See how a row of data is processed. You'll learn a lot and the mystery of migrations will disappear.