I haven't used ModernBill in years. I do remember it to be pretty "chunky". It's definitely a really large piece of software. I did use
WHM Autopilot for about 6 months. It wasn't bad, it got the job done. A lot of the big hosts prefer ModernBill over any other billing software simply because it has a ton more features.
It's quite expensive though, I do know that many hosts rent it and pay per month instead of buying a full license.