You must review the synthesis log files and confirm that all messages given by the tool match your expectations in terms of the design intent. Pay special attention to Critical Warnings and Warnings. In most cases, Critical Warnings need to be fixed for a reliable synthesis result.
If a message appears more than 100 times, the tool writes only the first 100 occurrences to the synthesis log file. You can change the limit of 100 through the Tcl command