20 % makes a huge difference for Debuff Teams.
My first Debuff Squad had Dahlia, Dragon Fruit, Bride Oncidum, Bipinna Cosmos and Water Lily in it (At that time i only had 3 Debuff Girls available) and they didn't last long in a prolonged fight. That team only reached 45 % Atk Debuff. It wasn't that successfull in Aqua Shadow Maps etc.
Since i have got Shrine Lycoris, Golden Lace and Cherry recently my Debuff Team is a Great Wall. They can tank damage pretty well in harder Maps. So Yeah Debuff Teams are High Tier (not as broken as Evade Teams but still really good) but in my opinion 70 % is necessary for it's success.
That's my Debuff Team atm:
http://flowerknight.wikia.com/wiki/Golden_Lace
http://flowerknight.wikia.com/wiki/R...Shrine_Maiden)
http://flowerknight.wikia.com/wiki/D...(Bride_of_Fos)
http://flowerknight.wikia.com/wiki/Cherry
http://flowerknight.wikia.com/wiki/Dragon_Fruit
I still want http://flowerknight.wikia.com/wiki/Snow_Drop for a full Rainbow Squad.
The bad thing about Debuff Teams: There are only a few good Gacha 5* available for this Archetype. So you will have to wait for a specific Gacha to increase your chance to get one.
The good thing about Debuff Teams: You can build a decent Team with those 5*. For other Archetypes like Crit/Solar Blast you pretty much need 6* for it to be viable.