What are the downsides of using bentonite for producing beer?

I have used betonite in wine, and I must tell you that I don't think I will ever use it again. Not only it clarifies, but it also stripes some color and flavor out, so be careful.

In fact, you should used it as a last resort if you are a having clarity problems, so it will also depend on which type of beer you are making. Racking is usually enough for me to get the beer clarified enough to my taste, but if you are aiming for a light and crystal clear beer (like a "coors light") it might work.

If you decide to use it, I recommend adding it after the completion of the fermentation (and after racking) because Bentonite requires to be mixed, and you don't want to move the sediments around.

I have not done bottle fermentation after using bentonite, but you can still add some yeast just before bottling (which might affect the clarity again), so forced carbonation seems the best option if you can.

There is use and overuse. As with all fining agents, overuse may drop too much yeast from suspension, leaving you with slower refermentation. It would be hard to totally stop refermentation, but I think it might be possible. If used moderately, I double the time I wait.

Overuse may also cause problems with head, I've read. But never had this happen or heard it happened from anyone I knew. Maybe we just didn't use that much?

Most findings make colour a little bit brighter. This may be either good or bad, but difference isn't big. I feel bentonite makes biggest difference, but I didn't have representative sample of batches to tell for sure.

I never heard or felt it to affect aroma of dry hopping or any other aromatic additions in secondary. It helps to leave them in the bucket when transferring to the bottling tank.