Sweet Spot?

Do all ribbon and electrostatic speakers have a small sweet spot?
From my experience, Yes.

I think the ribbons will extend the range a bit more. My Ininity 4.5's and my friend's Maggie 3.5's put out a wonderful sense of height and depth that more than one person can experience without being on the other's lap.

My Martin Logan's I love, but basically if you move out of the center the magic diminishes. It is definitely not for a group of people.

This is for real serious and involved listening. My friends and I enjoy the sound of both ribbons and stats without HAVING to sit in one area, and I can putter around and really enjoy the sound, BUT for critical and involved listening I would say basically, yes.
All electrostats have a small sweet spot? Absolutely not!

Many if not most ribbon and electrostat speakers have a small sweet spot, but definitely not all.

The Sound Lab electrostats I own and sell have extremely uniform radiation over an arc of either 60, 75, or 90 degrees, depending on the model. The tonal balance is correct from anywhere within the room, and the soundstaging is exceptionally good from off-axis because the Sound Labs radiate as a line source instead of a point source, so the SPL falls off more slowly with distance. As a result there's less loudness discrepancy between the two speakers for off-centerline listeners. When I give demos, at some point I have the customer sit up against the side wall, actually to the outside of both speakers. The soundstaging is still enjoyable from over there (though of course it's better up and down the centerline). In addition, the uniform radiation pattern makes for more correct timbre, but that's another story.

Sound Labs use a curved array of vertical facets, each angled at about 12 degrees to the facets on either side. Each facet is about 4.5 inches wide, so each individual facet does beam, but in the array configuration their patterns blend to give, as I said, extremely uniform coverage. There is no picket fence effect as far as I or anyone that I know of can hear (presumably there is some ultrasonic frequency where a picket fence effect kicks in, but it's of no audible significance).

The Beveridge line of electrostats also had a very wide sweet spot - again, they sounded excellent from anywhere in the room. The Beveridges used a physical lense in front of the panel (inside those huge boxes) to give uniform coverage over a 180 degree arc. The big Sound Labs cover a 90 degree arc, front and back.

Ribbon-type speakers typically give you a wider sweet spot than flat or single-panel curved panel electrostats, but I have not encountered a ribbon speaker that gives you as wide a sweet spot as the Sound Labs.

If you have any questions about any of this, give me a holler. I could go into more detail if you'd like.

My Apogee Duetta Signatures have a wider+larger soundstage than any of my other 3 pair of speakers. They do have a sweet spot but I can enjoy from many different seating locations. My room is on the larger side so this may help.

My Martin Logans have a pretty small spot, but, oh, what a spot that is;)
Some of them were designed more for near field listening; and have a tight sweet spot by design. I think the Innersound Eros are narrow on purpose..