Dare I really test it's capacity?
Last week I dared. I turned it loose on one of my collection folders with 175,000 fonts, some dups.
It ran for 9 hours! I now have 148,019 fonts in my library. Dare I add the other collection folder with about twice that number?
To switch from Active Fonts to Library now takes about 30 seconds. That is a lot of chugging, but MainType gets it done!

Scrolling through that many fonts as fast as possible will over-run the Preview text from time to time (on graphics intense fonts), but a few second wait allows it to catch up again.
This has allowed me to look at fonts I hadn't seen in at least 5 years and move newly appealing fonts into groups where I will at least see them again. WEll worth the test, and MainType passed with flying colors and kudos from me!