Usually any circuit have a working bandwidth and in this bandwidth the circuit should be loaded with the optimum load
for conditional sable amplifier, if loaded with 50 ohm it should work fine without any stability problem but in case you put a filter at the output, this filter have 50 ohm in the working band but have reaction out of band which can cause the amplifier to oscillate
for unconditional stable amp no problem what evver the load is in and out of band
good luck