No, there is a subtle difference between "pioneering" and "first".
In your example: "the first experiments", in all probability, did not allow us to do anything: in this sense, "first" means "original". "The pioneering experiments" are a wider group of experiments definitely organised and arranged to target a solution. There is a nuance to "pioneering" that indicates that the outcome will be in some way successful: the start of a 'success story'.
E.g. "The first experiments to establish the orbit of the earth around the sun ended with some very strange results, however, the pioneering work of Copernicus led Johannes Kepler to discover the elliptical orbit of the planets."