There are a lot of pressure (stress) put on the 5 sides of the tank. The higher the tank, the more pressure. Have you ever look at a large acrylic tank from the side? The longer the tank, the more likely it will bow unless it's using insanely thick acrylic.
Nothing to hold a cover or lights but you can go topless & hang lights or use legs. I decided I was too much of a klutz to use just legs over an open top. I put a couple glass strips to hold a cover but it's hard to decide how full to keep it without the frame as a stopping point, I almost overflowed it last time. Still I like the look.