The original MG-III etc. were criticized for the ribbon tweeter not blending. Like go_sox, I never had a problem with that, at least once they were properly set up in the room. I'll admit they are not great "near-field" speakers though mine are set up fairly closely now. The complaints I heard more often (as a tech at a Magnepan dealer as well as an owner; the 'net wasn't really around then) were about the bright highs. 99 times out of 100 (at least) the system measured flat, but people were not used to hearing a truly "flat" high end with as much sound as that tall ribbon would put out. Magnepan added the extra resistor and a lot of people preferred that sound. Me, I liked mine flat! I do have my current system shelved about -3 dB at 12 kHz, but that's mainly because I find much of the music mixed today far to bright and harsh, and I can't hear over 12 kHz anymore anyway.
I do prefer the larger panels, not only for their greater bass and output, but also the greater soundstage they provide.
I will note that the pulse response of Magnepan's has historically been excellent, and a lack of blend would show up in that test. I have not measured (nor seen) impulse/step response testing of the latest models but I doubt it has gotten much worse.
Room treatment is a must, imo, for any dipole (Magnepan or other planar dynamic, electrostatic, or full-range ribbon, plus those few using conventional dynamic drivers).
IMO, FWIWFM, YMMV, etc. - Don