There have always been advantages to steam engines, or external combustion engines, but they've been appearing in fewer and fewer applications because they are relatively bulky, complex and inefficient, hence uneconomical. For some applications, like moving ships and generating electricity, they are still the way to go, in situations where space and weight are not an issue, they can be made extremely efficient, approaching the theoretical Carnot efficiency of an idealized heat engine. They have potential environmental advantages because the fuel combustion can occur in a more controlled, low pressure environment and produce less toxic compounds. But this particular advantage is negated by designs which can burn multiple fuels, I can't see how they all will work equally well.

This idea is scientifically legit, this is not a something-for-nothing, perpetual motion scheme, but whether or not it is practical in a variety of applications, and whether it's advantages outweigh its cost remains to be seen.