The biggest issue is that the supplemental enzymes shut down the body's own production of those enzymes.
Any idea over what period? I was using digestive enzymes with each meal for three or four months and when I stopped my digestion seemed to continue to work well, not at all impeded.
A second problem is if the enzymes contain cellulase or hemicellulase.
I don't expect to have a problem with the (tiny?) increase of monosaccharide metabolites. I can't imagine my cellulose digestion increasing my Sugar digestion by a substantial amount.
The beneficial flora in the intestines ferment these fibers to produce anti-fungal acids, and pathogen killing bactericides and peroxides. Therefore, starving the flora by predigesting the fibers just encourages pathogen overgrowth in the intestines and candidiasis.
I haven't had problems with biome imbalance, so a reduction of cellulose reaching the intestines isn't too worrying for me. How much reduction of cellulose are we talking, anyway? Surely not a complete digestion of all ingested cellulose? Anyway, if I find I experience fungal or pathogenic overgrowth, I'll try to avoid cellulase. (Maybe there are enzyme supplement formulations that leave out cellulase? Why would they include celluses in the first place?)
So, my main concern would be "atrophying" my body's enzyme production. I'd love to look into that in more detail if you had any references you could point me to.