I'm not using an E-bike for touring, but I've got some insight due to the fact that my father offers commercial organized bike tours with E-bikes (see here)

That's where I also think it offers the biggest advantages: In a group, the use of E-bikes can help even-out differences in personal fitness. With E-bikes the group members can go at pretty much the same speed: The really fit participant can ride with a "normal" bike, the untrained partner can keep up with the group thanks to an E-bike.
With E-bikes a hilly terrain - like Tuscany or the Provence - is doable even for cyclists with a very limited fitness level or of advanced age. The E-bike opens up quite a few new possibilities, particularly for senior citizens.
Additionaly, in organized group travels some of the major disadvantages of E-bikes can be avoided: My father has a second set of batteries in the accompaning minibus. Like that, the range of E-bikes (which usually is below 100km) can be extended to 150km/day, which makes quite decent tours possible.
As E-bikes tend to have more defects (due to additional weight and speed), an accompaning minibus with a spare bike or two can also become quite handy.
In organized tours the traveler can also count on the fact that there will be eat nough plugs at the end of the day to reload the batteries.

This said, the main disadvantages of an E-bike as a travel bikes becomes obvious, particularly when traveling alone. The main attraction of such travels is the freedom and independence one enjoys.
With an E-bike, the traveler becomes a lot more dependent: The range is limited, if traveling without spare batteries (which are bulky and heavy). The traveler depends on the availability of a plug in the evening. Wild camping is not really an option like that. A defect can seriously impact a bike tour, particularly when no helping hands or alternative transport options are available.
As technology advances, the problem of limited range will probably vanish, and E-bikes might become more reliable. In 20 years or so I might also switch to an E-bike for my individual bike tours...

Yours
Markus