Right now, a non-hybrid, non-turbo, non-DI gasoline engine wins handily in the complexity department. There are fewer of these left (all three of the Equinox engine choices are turbocharged DI engines, two gas, one diesel). Plus, the few that are left are still equipped with complicated and often fragile (Honda comes to mind) vario-valve/cam stuff.
There fuel economy is abysmal. And because they do not have the extra power afforded by other technologies, the only way they can get them to have a chance to actually move whatever they are in is to make them BIGGER. Add to that the fact that they are almost always saddled with an automatic transmission, you can see why they are at the bottom of the "wanted" list until the buy in price gets brought into the equation.
But by the time these relatively simple engines are completely gone, the gasoline engines will have particle filters and some form of SCR added to them as well.