hi agree with CiscoRQ i'd like to take this a step further to discuss the concept of the "unassembleable".
In my work in a new type of library service where my colleagues and i engage ordinary people (and try to document) experimentation across the in arts, science, technology and enterprise, we often come up against the gaps in english. There are interesting issues with language when working across a range of technical disciplines. In this work we need language to keep pace with what we are doing conceptually to communicate with collaborators and move the work forward.
On a practical level "Assemblable" seems pretty legit use of grammar to me and so should be recognised. I'm no word nerd but making a compound word out of the verb "assemble" and the suffix "able" seems pretty simple to me and kind of what suffixes are made for. The common reflex in our lab/workshop when English can't deliver is to turn to the German/ Swede in the team. German (and Swedish) seem to be really good at stacking words together to communicate meaning.
To me this reflex of looking to another language, signals a possible failure of english to adapt.
The requirement for a word to describe the assemblability of a design is brought into sharp focus when a design can not be assembled. A Design that can not be assembled is a failed design.
The shared experience in my workplace is that there is the regular problem that can occur when designing a multicomponent object (especially in CAD applications) Its pretty impractical to design a component that can not be physically incorporated into - assembled - into a target assemblage.
It is an underlying requirement of component design process that "the design" (noun) needs to allow for the practicalities of the assembly process, especially in the design of components for a multi component assemblage. With regards to the application in a CAD application we are talking about mechanical or industrial design. But this also applies to the design processes (developing an practical solution to an identified application) in a range of disciplines - process design, even language (meta)
Yes this may all be argued to be niche application, but i dont think it's necessarily a niche use of the prefix "un" or suffix "able". These are stock stand uses of suffix/prefix and the rules for incorporating new concepts into mainstream use suck if I have to go to a different language to communicate this pretty simple idea.