Wicca and Druidism had to be made up from scratch, as there was just about zero reliable historical anything.

With Wicca, Gardner's claim to have been initiated by a secret coven of Celtic witches, is, of course, like all such claims (golden tablets, etc), pure bullshit. That it is all made from whole cloth doesn't bother me, it serves the people practicing it, and it's pretty much harmless. Wicca has split into "British Traditional" (Gardnarian) and sects that were not trained in the Gardnarian tradition, primarily in North America and Continental Europe. This is fine - it's a developing philosophy and some of the bullshit is being thrown away. Of course, there's Dianic Wicca, which accepts only naturally born lesbians and recognizes only a goddesses, and other splinter groups - pretty much harmless because of very limited membership. There are plenty of other covens for the rejected.

Contemporary Druidism started out with just about no reliable information except that the Druids believed in reincarnation and had some to do with mistletoe. Everything else is questionable due to having been written by their enemies, and some just by hearsay. It started out monotheistic, based of Freemasonry, but evolved rapidly in the 20th century as archaeologists found more information on Celtic culture. It is now polytheistic and nature based with a strong environmental bent. Again, I don't see a problem - it serves its practitioners and is harmless. Like Wicca, it's highly decentralized. Of course, they like to have events at Stonehenge, which was built 2000 years before the first Druid by a people we know nothing about except they were very good at astronomical mathematics and at moving rocks.

Reconstruction is much easier in places like Lithuania and Abkhazia where much was preserved, and even Russia and Ukraine. The largest Pagan temple in the US is Russian. Ossetia is a good reference for reconstructing Pagan traditions, because 30% of its people still hold to an unbroken tradition, having rejected both Christianity and Islam.

The main problem is in Germany, where violent neo-Nazi groups are disguising themselves as Heathen churches.