Bipolar transistors suffer from the thermal runaway of parallel devices you describe, but not MOSFETs. MOSFET on resistance increases with temperature, so with

Bipolar transistors suffer from the thermal runaway of parallel devices you describe, but not MOSFETs. MOSFET on resistance increases with temperature, so with multiple devices in parallel the hottest one will flow the least current. This is an important effect even within one device, which is actually an array of thousands of junctions.