I have used HOBs in conjunction with under gravel, because I like to have a lot of filtration.
When I purchased my current 45, I want an under gravel for it, and the owner of the lfs scoffed at me that I was out of date with the times. ( ok, whatever…). So, I pulled the UGF out of the 30 gallon tank that I was upgrading from and put it in the 45. It doesn’t cover the entire bottom, but it does cover about 2/3 of it, with a power head to drive the turnover. And, I have a HOB in addition to that. It works great!