There are several Eichler neighborhoods near me. A great design IMO but badly in need of modern materials. The thinly insulated roofs, single pane windows and radiant heat systems are a problem on the original Eichlers.
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“built like crap” but that is not to denigrate them; they have huge single-pane, untempered window area and as you say, outdated insulation and heating systems.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.