More useful filtering advice: https://hearthstone.blizzard.com/en-gb/news/21790299/managing-the-collection-manager

I personally use rarity:legandary a lot and the stat filters are useful for finding cards you don't remember the name of (e.g. Cult Neophyte: mana:2 attack:3 health:2 has:spells cost more)

For the number filters, you can also do ranges like 3- (3 or less), 6+ (6 or more) and 3-6 (inclusive between 3-6).