'High aspect ratio grain' = T-grains or other methods to produce flatter halide crystals (e.g. Delta gets to the same end by different means) - as opposed to '3D' crystals that have greater depth - they seem to be better terms to use because the structures are often more complex in shape than simply 'tabular' or 'cubic'. If T-grains (in particular) go round too tight a bend on too thin a support, it seems that crystals above a certain size may be prone to 'cracking', causing fog. From what I recall, Tri-X in 120 was coated on a 3.6mil triacetate base, Ilford uses 4.33mil triacetate in 120, Tmax 100 uses 4.7mil triacetate (unclear if 400TMY-II needs same base thickness - or if it and 400TX have both gone to the same base as Ilford (I think that Portra definitely is) - which would make sense from a supply perspective & possibly clarify why 100TMX in 120's price jumped more than the others) and Foma uses 4mil polyester (as far as known).
So is it the fact that it is polyester as the base that makes it the problem for T -grains or the fact that it is 0.3mil less or a combination of both?
It's in their own data sheets that their materials have overall lower MTF performance (less apparently sharp - especially at low frequencies - which will give Foma materials a more 'vintage' look), worse granularity (hence lowering total information capacity) & require speed increasing PQ developers (Microphen type) to reach specified shadow speeds.