Sure you can burn them, doesn't mean you're accomplishing anything from a thermodynamic point of view.

Metric to look at is EROI - Energy Return on Investment. How much energy do you put into growing these crops to get a unit of energy out?

Corn ethanol in the US is stupid because it's a 1.6:1 return at best, often a 1:1. To get a unit of energy out of corn ethanol, you need to put almost a unit in. And that's economic inputs, we know the sunshine is free. Sugarcane is a bit better, but you can only grow it in the tropics.

For reference, solar tends to be something like 4:1 - 10:1, wind is 10:1 +/- onshore or offshore, fracked oil tends to be down around 5:1-8:1, and a saudi well is (or at least used to be) 40:1.

All too often those have bad environmental impacts of their own. Coconut oil ends up being grown in plantains that require clear cutting natural forests. The agricultural process to plant, grow and harvest sunflowers produces significant CO2 just as it does for growing corn for ethanol.

Good point, liquid and gas fuels will continue to be important into the future where energy density is needed, fuel cells could be a good option also.

Depending on the engine, perhaps. But this still releases CO2. This is not about supply limitations.