Why shouldn't I mix affinities within a Dragonfire army?

Because the troop type is unique per army: all 3 dragons carry the same one (a single selector). That type triggers your research for that type — the biggest damage multiplier in the game.

The trap: picking a "compromise" type to please three dragons that each have a different affinity. The result is one or two neutral or red dragons, losing their +20%.

The right method:

1. Pick the type you've researched most (a big research gap wins).
2. Fill the army with green dragons for that type.
3. Only if two types are close in research does the +20% affinity break the tie.

Don't build an army as an assortment of your best dragons across all affinities. Build it as a single-type team where everyone is aligned. The example roster has 5 Spearmen-green dragons — that's a natural axis.