From a 2014 perspective, they are “built like crap” but that is not to denigrate them; they have huge single-pane, untempered window area and as you say, outdated insulation and heating systems. Those were the standards (or maybe I should say lack of standards) extant at the time.
The problem(s) arise should you go to update them. Many local building departments—especially hoity-toity California ones where many of those homes were built—will insist you bring *everything* up to current code should you start fiddling around. There is almost no limit to the pain they can inflict upon you should you get so involved.
My sister owns a California Modern but I don’t think it’s an Eichler. It has the open I beam ceiling and wall of glass to the backyard. The roof is thin but not flat. Hardwood rather than concrete pad and no radiant heating. It’s a nice, bright house.
Too bad it’s in that part of California that is being handed over to Mexico but that’s how the GOPe and comrade Obama want it.