If it's the "rigid" part that you're wondering about, I found that our rubbish wall-mounted fan was more effective when I replaced the corrugated duct with smooth, rigid ducting. Also, I've never worked out how you deal with dust buildup in an inline fan. It certainly builds up in corrugated ducting and on fan blades, which reduces their efficiency slightly.
If it's the "rigid" part that you're wondering about, I found that our rubbish wall-mounted fan was more effective when I replaced the corrugated duct with smooth, rigid ducting. Also, I've never worked out how you deal with dust buildup in an inline fan. It certainly builds up in corrugated ducting and on fan blades, which reduces their efficiency slightly.