why not undergravel

I've heard that undergravel fiters are bad for plants and bottom
feeders, but I haven't heard why.

I put an undergravel filter in my 55 gallon tank because I knew
that it would be a nightmare to put it in after setting it up.  
I also have a charcoal filter with an intake tube and a reservoir 
which appears to be pretty standard these days.  

My spotted dojo and my 3 coreys seem happy.  My plants are doing well.
I have really deep gravel.  I keep the air to the undergravel 
filter cranked down pretty low anyway.  I appreciate the sort of 
filtering it does, and it seems to help get food down to where the
loach and the coreys want it.  The only other fish are columbian tetras,
and they don't seem to mind.

So my plants are 2 swords, 4 moneyworts and 2 of these dark
evergreen-looking things that I don't know the name of.  All seem to be 
doing well but it's really too early to say that.