Of course it has scientific basis. The scientific basis is that electromagnetic fields alter biological processes, thereby it is possible to produce effects in both animals and humans.
And if you deny that electromagnetic fields can alter biological processes, then you really completely deny science.
In addition, EHS is really a poor choice as a descriptive term for these symptoms. It is no different than saying that when someone punches you repeatedly, you develop human fist hypersensitivity.
It is a misunderstanding that people who suffer from EHS could somehow sense microwave radiation directly. This is impossible, because humans have no direct sensing apparatus for this type of radiation.
What happens is that they feel the secondary effects of the disruption in their biology. For example a change or disruption in the ATP production of the cell can lead to many secondary effects, as can can the effects on cell wall permeability, just as two examples.
Trying to do provocation tests on human subjects, only really shows a misunderstanding of what is happening.