The ICE in general is pretty wasteful compared to most other engine types. As said earlier, what's going for them is their simplicity to maintain.
The difference I guess, and reason they aren't totally phased out in the clans is because you can burn almost anything in them. That makes them extremely useful for utility purposes like support vehicles and industrial mechs. I can see why they'd use a few on combat vehicles too, but I think the overwhelming majority of Clan combat vehicles use fusion, except for maybe the Hells Horses. They actually went out of their way to make chemical lasers relevant because they didn't want to use fusion engines, and didnt want to have to add capacitor banks to every tank they make.