Yes, some loans are effectively free money, and the ratio, and where you borrow from, are portfolio issues. Even with a million net worth right now, you'd be crazy not to have a 30 year fixed mortgage IMHO.
Problem can be that people don't actually offset the debt with investments. They use the debt to increase consumption. Classic example: living in a too big and expensive house because 'rates are so low, and the interest is tax deductible'. That's bull. The house you live in is not an asset. But owning it may give you the option to borrow quite a lot of money at artificially low rates.
In the end it's all about your net asset vs liabilities, and cashflow and portfolio yield.