Muzzle brakes generally require there to be a bit of muzzle blast to make a difference in recoil or muzzle rise. With the length of barrel on an SKS, most of that's already done and there isn't much flash left to deflect. Longer barrels help reduce recoil by their greater weight and more time for the powder to burn and less blast out of the muzzle.
Brakes usually reduce recoil at the expense of noise. On most guns, I would rather have the extra barrel length that a brake would take up instead of a brake. Now flash hiders are another animal. I've had good experience with a few, but they were on shorter barrels, ie a 16ish" Saiga in 7.62x39mm and a 16ish" G3 in 7.62x51mm. The G3 with a longer barrel and a ban-era muzzle brake had a horrendous muzzle blast and noise. With its current flash hider I've found it much more enjoyable to shoot.
