I wouldn't say horribly inefficient. Most polycrystalline silicon cells get 12-15%, which isn't too bad. Some research groups have made PV cells that get 40-60%. The problem is that they don't do direct fuel conversion - you have to then use the electricity to do electrolysis or drive other chemical processes, which are inevitably going to be lossy. So you get 12-15%, run it through two or three processes that are 10-50% efficient, and suddenly you have 1% or less solar-to-fuel efficiency. Which, yeah, totally sucks.
And solar concentrators take a ton of space. This process runs at 1500 celsius or higher, so you can't just do it in a trough. You need a tower or dish system, which take up huge amounts of space.