I've just stumbled upon this having had cause to research the word myself.
It's not something that I'd use myself as I think it sounds clumsy and, as has already been demonstrated here, there are better alternatives. But, I'd argue that it is a perfectly valid word.
An ngram viewer search shows it (and 'ununderstood') in use for the last 200 years with many of the examples coming from some fairly heavyweight sources. Schopenhauer is a name that springs out from the list as a user.
For me though, the OED is the ultimate arbitrator in questions of legitimate usage:
a1631 J. Donne Serm. (1958) IX. 51 Let him have
known..In-intelligibilia, In-investigabilia, (as Tertullian speakes)
un-understandable things, unrevealed decrees of God.
a1843 R. Southey Common-place Bk. (1849) 2nd Ser. 251/2 The vile
and ununderstandable Machabeo he ranks with Homer and Virgil!
1872 B. Brierley Cotters of Mossburn xxiv. 252 There is something
very ‘ununderstandable’ going on between Luke Brundrett and Miss
1891 H. Herman His Angel 46 He stammered a few un-understandable