What are your needs? Are you treating CPF as retirement funds or...? For myself I separate CPF from cash investments. Still part of portfolio; but not the only one as you know CPF have certain criterias for withdrawals.
Investing in robo advisor on monthly basis helps you to spread out the risks (sometimes you buy when it's good and sometimes when it's bad.) For CPF, they give a good stable returns but you have to meet the FRS.
Robo advisors you can easily cash out if something happens and you need the money (not that you should invest if you don't have an emergency fund but just in case..) while CPF is for a different purposes altogether.