My understanding is that in addition to extra weight and difficulties with cooling, aerospike engines do worst around Mach 1-3. Also, they aren't steerable. The

My understanding is that in addition to extra weight and difficulties with cooling, aerospike engines do worst around Mach 1-3. Also, they aren't steerable. They need to either be throttleable to provide attitude control or supplemented with steerable thrusters like Soyuz. Both bring additional weight penalties. In short, like almost everything in rocketry, the devil is in the details, and a percent of two of performance either way can make or break a design.