In a nutshell, the Server Edition is limited to 2 simultaneous file backups and 2 deduplication workers.
The Server Enterprise Edition doesn’t have those limitations and also offers Granular Backup (backup files internal to VM from the host) & Granular Restore (restore files from within VM backups) features.
The limitation of workers and simultaneous files basically causes backups to run slower on Server compared to Server Enterprise Edition.
The two editions are increasingly drifting apart to satisfy the needs to different groups of customers; hence, different new features are being added to future versions of each and may or may not be added to the basic Server Edition.
Check our Buy page for a comprehensive comparison chart.