fulminis, think of the problem in its basic form. What is a broadband absorber? It is a velocity sensitive absorber. What does that mean? It means the absorption of sound energy is due to resistance to gas flow through a porus material. What does this require? Free flow of the gas through the material for maximum benefit.
It gets a little confusing because people tend to place broadband absorbers near room boundaries. More on that in a sec. The alternative as you note are Helmholtz resonators, which are pressure based absorbers. These by necessity must be place at room boundaries where pressure is at a maximum and velocity is near zero.
Back to the confusing with velocity based broadband absorbers near room boundaries. Notice that key word "near." Since all frequencies will have a pressure maximum and velocity minimum at a boundary, we know that velocity for all frequencies will increase as we move out from the wall, to a maximum defined by their wavelength. A broadband absorber might well work in the middle if a room, keyword might. If the problem frequency in your room happens to be at high velocity in the location if your absorber, great. But it could be that there is a velocity null there. Since broadband absorbers by virtue of being broadband are typically shotgun approach without explicit knowledge of precise problem frequencies and their maxima/minima mappings, we choose to put them where they are guaranteed to work some, even if not optimally. So "near" boundaries. Corners are nice because they are near multiple boundaries, and thus work on multiple axial modes simultaneously. But only near, which is why the recommendation to space them off the wall a bit if possible, and/or use a thick chunk if absorptive material.
Oh, and I suppose most people would object to a chunk of fiberglass in the middle of their room!
Back to risers. Broadband absorbing risers are just corner traps that can support some seats. Both are broadband, and are placed near room boundaries. Would you think it was odd if someone suggested you cover a corner trap in plywood and cut some holes in it near ceiling and walls? Then why would we do that to a riser? Bottom line, you want your riser to mimic a corner trap as much as possible. That means making the wood as much "not there" as possible. And that includes sides and front, so it can act on multiple axial modes at once.
Some do intentionally design for Helmholtz resonators or damped versions of them. I believe Erskine's group usually does this. They also have tons of experience doing so, and all the simulation tools and measuring tools necessary. Most diyers do not.