On one of the last such tests done on a widebody in the US, a volunteer was paralysed for life.
Be careful what you wish for.
This is the reason why in many countries there is no more spin training or single-engine-out exercises during check rides on twin engine GA aircraft.
The number of accidents during such training intended to prevent future accidents made it questionable.
However, if we accept that it is too risky to test it, how can we rely on it to be a safe procedure in case of an emergency? Giving the message to all crews that an evacuation is too risky to test, how do we expect them to take the decission in a real emergency? Just compare with the discussion for the BA in Las Vegas.
Being able to safely evacuate an aircraft is one of the basics in aircraft design and certification. So we should make sure it really works.
It is quite encouraging to see, that in the last 20 years it did. It is the big question whether the situation has become worse since (more carry on, more uneducated passengers...).
All recent accidents showed that:
- there are high risks, passengers do not follow the instructions, crews do not always take the best decissions
- it works
Passenger video show rain increasing as the rotate. Occums razor! Microburst or severe wind shear.
Or engine failure/flameout due to heavy rain... Engines are tested to accept heavy rain in flight, we do not assume that pilots would take off in such situations, we do not design engines to accept any ammount of rain at take-off power, potentially also ingesting additional water from the nose wheel spray.