My understanding is that ETS is only used as a last resort and that most of the drawbacks/risks occur at the time of the surgery. I have yet to read about any long term risks and it DOES NOT destroy "a lot of the autonomic nervous system". It is microsurgery and typically only involves destroying one to two levels of the thoracic sympathetic chain.
It has been used for over a decade with excellent results and minimal acute complications. No where in my reading did I read about any longterm complications that would hurt a sportsman.
I am sure Amer was counseled properly.
From reading Amer's blog/website he was well aware of possible negative side-effects so it was a conscious decision to have the surgery.
There have been very few thorough medical studies on the subject. The surgery is notable for commercial private doctors in the US that have been actively solliciting patients. There has been a backlash against this surgery by people who suffered long-term side-effects that have started a large number of "warning/support" websites against ETS with full description of the long-term negative side-effects.
The Swedish doctor who pioneered the surgery stopped doing it and has now dedicated his career to tying to help people who suffer long-term side effects, and it has been banned in Sweden.
When thinking of possible drawbacks to a sportsman, I am considering for example the fact that the surgery destroys the sympathetic autonomic nervous system connection to the heart, which could interfere with the regulation of heart rate for excersize (sprints etc).