Two-piston hydraulic disc brakes are a solid choice for many mountain bikers—they offer smooth, controllable stopping power (called modulation) that's more than adequate for XC racing, light trail riding, and moderate terrain. If you're a lighter rider or mostly ride rolling trails without long, steep descents, 2-piston calipers will serve you well.
Where 2-piston brakes fall short is on aggressive terrain. If you're hitting steep, sustained descents, riding enduro or downhill, or you're a heavier rider, the extra clamping power and heat dissipation of 4-piston calipers make a real difference in consistent braking performance.
The bottom line: if your riding stays on the milder side of mountain biking, 2-piston hydraulics keep your bike lighter and simpler while still giving you reliable, low-maintenance stopping power. But if you're pushing into aggressive trail or big descents, step up to 4-piston for the safety margin.