We need a communications theroy expert here to explain the limitations and tradeoffs of spread spectrum. I'm not that person. I know enough to know it is an enormous subject, very complicated, non-intuitive, and SS is a generic term that refers to many different modulation schemes, not just one. SS is not a cure all. It will not eliminate the possiblity of losing your aircraft due to a weak signal. It will (depending upon the multiplexing technique) make beign 'shot down' much less likely.

The tradeoff, I think, is that range will become more of an issue. This is mainly due to path loss, but also, multiple simultaneous SS users will reduce range. For example, 4 simultaneous users would reduce the range of each user by a factor of 2. Multiple users do not interfere with each other directly but increase the noise level for each user.