The author is trying to avoid the word "unpredictable" here, because prediction is possible in some cases. "Inaccessible" presents the same problem. If I have to translate the Russian word "труднодоступный", in an informal text I might use "inaccessible", but where precision is required, e.g. in a patent, I would have to translate it literally as "difficult of access", even though I think this sounds clumsy in English.