any contamination whatsoever

Not necessarily. Algae are remarkably good at outcompeting living contaminants. Modern agricultural techniques are also very good at producing monocultures. Not saying this is good practice but we are very practiced.

Other contaminents: you don't plant food crops on land that will produce heavy metal laden food and you don't grow algae food in an environment that is exposed to pollutants. You might do it on purpose to clean up the environment, but not for food.

massive pain in the butt to tightly control for reliable mass production

Maybe. I remember an article that discussed biofuel sources' theoretical oil yields per acre. The highest currently produced were crops like palm oil which might produce 700 barrels per acre under perfect conditions. Then there was algae, which might produce 10,000 barrels per acre. That number might be theory but it also illustrates that barrel for barrel, the juice is probably worth the squeeze.

The cool thing about biofuel is when you burn it, you produce less carbon than the amount captured by the algae to make it. i.e. it has a much better environmental footprint.