I’ve used some formulas, shimano, hope and hayes. I LOVE my 3 1/2 sets of hayes stroker trail/carbons. They are very very cheap second hand and very easy to bleed. I believe its the same process as avid. Only problem I have had with them is the piston in the lever is prone to wearing and wobbling however this isn’t such a problem on the trails and carbons as opposed to the rydes. I use a set of trails on my MTB tandem with a 200/180mm rotor setup. They work great for a <300 pound team and a >400 pound team.
Hopes were okay but lacking in power (M4s), formulas are bonkers but very little modulation for me as they were probably to powerful. The shimano i’ve used were good and nice and reliable in terms of stopping power, feel etc but I hated bleeding them with a passion.
IME
Hayes – good power, feel, easy to bleed.
slight lever wobble.
shimano – pretty good power, reasonable feel.
hated bleeding.
hope – okay power, lots of modulation
never wanted to bleed them given how complicated my brother made it
sound
formula – crazy powerful
not much modulation but they were to big for my weight. easy bleed if i’m correct.