It's a problem because it's impossible to keep up either everything. Joe buys the Complete Book of Ducks which has the Quack Them Up spell. As a DM I know have to buy, borrow or double check at the table wasting time. Joe has a tendency to read things in his favor (not necessarily on purpose), so I like to double check. Then Kim gets The Greater Goose so she can play the Glorious Honker bard.
But it gets worse. Because there are so many feats, spells and combos there are trap options and incredibly OP option. Combine the Birds of a Feather feat with Quack Them Up and a Glorious Honker, throw in My Little Chickadee and suddenly you have an unstoppable gander. Don't even get me started on the combos if we throw in Glorious Groundhogs.
I could go through and weed all those out but I'll have frustrated players and I waste a lot of time and money as a DM. That, and because they needed more books they published Radical Rats which is just garbage.
Last but not least, people in all roles are just overwhelmed with the options. The paradox of choice makes it so they don't know what to do.
There are a lot of issues with bloat IMHO.