That's odd because Exaktas tend to be very robust. With the exception of the clockwork for the slow speeds and self timer, they're very simple cameras. On the other hand the last ones, the VX1000 - the quality was really bad. The 1950s Exaktas are made about as well as anything, but by the 1960s Exakta apparently was competing on price instead of features. I think the VX1000 I had may have been the shoddiest SLR I've ever laid hands on. At that point I think there may have been a lack of pride or interest in the product as Exakta was enduring a lot of legal troubles with the copyright holders in West Germany.
I don't know if I would put Miranda in the budget group. The Sensorex cameras were very advanced and very expensive when they came out. Miranda also had one of the largest lens line ups, from I believe, 17mm to 500mm. Prior the introduction of the Nikon F, the Miranda was Japan's "pro" SLR, and even for a couple years after Miranda competed with Nikon. When Nikon came out with the 21mm lens (which needed MLU to work) Miranda came out with the 17mm lens the next year - and it worked with reflex focussing!
However Miranda was a small company, never made their own lenses, etc. etc. - IIRC Soligor bought them up in 1968 or 69, and managed to kill the whole operation in less than a decade (apparently through gross mismanagement).