I agree with Publius to a point. Unionizing in the sharing economy is moot. The idea of the sharing economy is to supplement income by renting out otherwise unused capital. The Unions are necessary where control of capital resides in too few hands and many fungible hands (unskilled labor or labor where the skills are abundant) are needed to operate that capital. This is an unequal balance where there is only competition on the labor side and the capital side is monopolized or nearly monopolized. A properly functioning market requires a balance of power between both sides.
In the sharing economy capital resides in many hands. If Uber succeeds in eliminating its competition and going all driverless then they become a monopoly and need to be broken up like any monopoly to create competition which is the backbone of a market economy.
I also agree that the Unions were slow to realize when they had overreached, just as the capital owners had overreached and gave rise to the Unions. Too much mistrust and bitterness between both sides led to a breakdown in the proper functioning of the market. But even more importantly the nature of the work changed over time and the Unions failed to adapt. In Germany there is less divisiveness between labor and capital. The two work together. In our country it’s always been a fight to the death.