Pros and cons of a single supermemo collection

There is no good and right about having one single collection or several for SuperMemo. You may decide when it is best to start a new collection, merge collections, or transfer branches between different collections according to many factors and current needs.

Interleaving

Video/audio repetitions break the flow

Stats

Averages may be not representative

Backup

Using the built-in backup solution is increasingly inefficient as collection count grows. You would need to backup each collection one by one. It makes close to no difference if you are using a git solution for backup, as I do strongly recommend to do.

Collection parameters

Same collection parameters for different difficulties

Final Drill

Having multiple collections make the selective final drill very accesible, as you can turn on/off the finall drill for each collection. For instance I could turn it on for language learning collections. Having a single collection makes the finall drill a everything or nothing choice. There is a word-around by creating subset reviews from the drill queue (video).

Priorities

Requires more precision for prioritization