In this game it seemed that when dregs used ronin, he tended to win the engagement or trade better. The buoys just keep missing and a buoy has to kill 3 ronin to be cost effective. Why not spam only ronin?

I guess in tests, the ronin only do well if they are well spread out. Too many ronin in a close space can't dodge and most die quickly, followed by a phase where the numbers are thinned and it takes many volleys before each one dies.

Phantom is super effective against Bouy, it only needs ~2 shots to have almost made cost.

Hammer can also usually hit grouped bouy because its slow.