If you have $3000 rent due, you can keep $3000 in checking to pay that, but you should not put that money in the stock market, since it could be worth only $2700 at the due date. Therefore I think the statement "We offer a checking account alternative that lets users invest their balance (not FDIC insured), otherwise it works just like a normal checking account" invites trouble. People should use checking accounts primarily to pay bills, so risky investments do not belong in them.
If you are really aggressive, you only need to keep money in your checking account between your payday and the due dates of your rent, utility etc. Everything else can be invested instantly on your payday.
If that amount of money is significant to you and you think you are missing out, you should stay away from the stock market. You are basically on the verge of bankruptcy, especially if you live in a country without proper social security (e.g. the US).
I like the idea if you are saving for something you do not really need, e.g. your second yearly vacation or some vehicle.
Thanks for highlighting that the investments can lose significant money, and that this is something to watch out for! Completely agree here. I do think that there is subtly around risky investments not belonging in a checking account. I agree that if you are in a situation where you live paycheck to paycheck you would only want to invest in very low risk assets (say US treasuries). That said, if you have some money saved up, you can invest your checking account balance more aggressively and still stay liquid even in really bad downturns.
Nitpick, but "US treasuries" include long-term bonds, which can fall substantially in value if interest rates rise. So low-risk short-term assets are confined to things such as Treasury bills with duration less than 1 year.
