Montaigne is only half gas
If, as Montaigne suggests in ‘Of Experience’, there is ‘no quality […] so universal as diversity and variety’, then the pursuit of common features is futile.
There are absolutely common features within and across Montaigne essays. The following excerpt (“I do not portray being, but passing”) doesn’t relate to an inability to characterize. Yes, he can be a self-evolving man of gaseous opinions without stable form, but that does not mean his writing composition has no stable form!
The words don’t shift languages or turn into three-dimensional beasts that claw out the page. There are stable patterns, of course, of language and grammar, but in composition too: across every essays does he not wrote from Experience? Does he not weave in References from his commonplace book? Does he not tend to explore two sides of an Argument?
Regardless of the organic endless digressions, aren't they made of Paragraphs, and doesn't each one have a start and end, a Frame and Finale? Are there not features of his Tone? Does he not bend Perspective to write about writing? And are the muses not present with Rhyme, Rhythm, Repetition, Imagery, and elegant Diction?
Where is the futility? Yes, the forms are ever-new, ever-changing, as are his ideas, but that doesn't mean he's compositionally undefinable.