Liz, I am just a relative newby and have only had my flow through wheelie bin running for a couple of weeks. However I got to thinking about things.
As soon as I started my wheelie bin with worms and bedding from my COWs, I noticed they could handle a lot more food. Now the fact is that the worm population didn't magically increase, so perhaps they are spending less time revisiting the bedding and lower levels, so the castings would not be as fine.
I notice too, that the WB is more tightly packed than my COWs, even at the top, I can pull growing seeds out of my COWs - roots and all, but in the WB, they break off at ground level. My WB is only about 16cm (6"

deep, but I would think that at 2 feet there would be a lot of pressure at the bottom, coupled with being dry, there would be minimal activity down there. On the plus side, it should mean no worms in the harvest.
If I build another one, I am tempted to locate the grill much higher up, mainly to allow a bigger access hatch and also because I don't think a deep feed/bedding layer is needed.