Yes, there are many people whose income is not stable, and sometimes they have to face problems when investing. A part of the income can be invested consistently. Especially if you start with a small amount of money, it is not bad. You should start with a small amount of money first and later, if you invest with a discretionary income for long-term investment, the investor can build a large Bitcoin portfolio.
Am thinking otherwise here, because you don't necessarily need to be stable financially for you to invest in Bitcoin and be successful, all that is needed for you to be able to invest in Bitcoin and be successful while holding for a longer run is to have a good money management skills, because even though someone is receiving a salary either weekly or monthly, if he can't manage his finance very well, he will not even have a discretionary funds left to invest with.
Because I have seen those guys that are contractors that doesn't that their income is not stable, but they still manage to invest with the discretionary income they were able to figure out from the contract they won, so it's more of good money management skills than being stable or not.