ESL's and wideband amps were a mess in the late 20th century. I have not been in the business since then so no data.
I forgot about the Hill Plasmatronics tweeter, another amp-buster. For that matter, IIRC all the early Quad ESLs (do not know about the current models) self-protected by shorting the amp inputs and a fair number of amplifiers became quite unhappy about that.
The original Apogee ribbon speaker was mostly resistive but at 1 ohm caused more than one high-end amplifier to overheat (or worse).
The Magnepan ribbon is below 3 ohms but again almost purely resistive.
Are you sure that's correct about most being sub 1 ohm. It's not been on the searches I have performed.
I guess we can debate "most". Most of the ESLs I have known/recall from over the years dip below 2 ohms at 20 kHz, varying from just below 2 ohms to about 0.2 ohms depending on maker and panel size. I did not do an exhaustive search and, while I was looking at a 0.2-ohm ESL yesterday or whenever, I didn't bookmark it. Here are some current ML models (below).
Martin Logan ESLs
Neolith: 4 ohms, 0.43 ohms @ 20 kHz. Compatible with 4, 6, or 8 Ohm rated amplifiers
CLX: 6 Ohms, 0.7 at 20kHz Compatible with 4, 6, or 8 Ohm rated amplifiers
15A: 4 Ohms, 0.5 at 20kHz Compatible with 4, 6, or 8 Ohm rated amplifiers
13A: 4 Ohms, 0.7 at 20kHz Compatible with 4, 6, or 8 Ohm rated amplifiers
11A: 4 Ohms, 0.6 at 20kHz Compatible with 4, 6, or 8 Ohm rated amplifiers
9A: 4 Ohms, 0.8 at 20kHz Compatible with 4, 6, or 8 Ohm rated amplifiers
I could not find an impedance for Sanders. Sound Lab only listed "8 ohms nominal". Acoustat spec'd "4 ohms nominal". I don't remember early Quads (57); Stereophile tested the 2912 and it dipped to about 3.3 ohms. I know Levinson targeted the 57 in his HQD (a system I heard several times back then) and they taxed his amplifiers. Quads after the original model or two have a network to control dispersion that also increased HF impedance so they are by and large an easier load than most ESLs (again if memory serves). Don't recall KLH, ServoStatik, Beveridge, etc. impedances and too lazy to dig (and I need to quit this and get practicing -- performance in a couple of weeks). When I read through the ML specs they didn't strike me as odd so I never really thought about it.