Aluminum is an ok choice for water holding...
They cost builders less than plastic tanks--at least they used to...and unlike aluminum waste tanks, average lifespan is at least 20 years. Most builders used aluminum for both 20+ years ago, 'cuz plastic in those days wasn't anywhere near the same quality. There are still a few holdouts who either don't trust plastic or get a better deal from an aluminum fabricator--and even a few who mistakenly think SS is superior to both aluminum and plastic...but as the quality of plastic improved and the cost came down, most builders have switched to plastic for both water and waste tanks in the last 10 years.However, the short answer to your question is, it's mostly only about cost. If aluminum tanks cost a builder less than plastic, they're gonna use aluminum tanks...'cuz as long as the tanks outlast the warranty--and they do...even a waste tank isn't likely to spring a leak for 2-10 years--that's all that matters.