Should a beginner investor who hasn't invested but wants to start investing now consider REITs? Is this too risky a financial asset class to start investing in? Or should I consider more passive ones?

The more prudent way then would be to choice REIT ETFs.

Either one of the 3 S-REIT ETFs (with SG:CLR havng the highest true Singapore asset allocation.

Or more risk free, diversified into those

ETFs I mentioned at the end of my text here:

https://seedly.sg/questions/what-is-your-genera...

Do not follow single stock/bonds/REITs investing, so many academic studies demonstrated that that doesn't work, even Warren B., most cited investor, failed over the last 10 years and (integrity !) admits it openly and recommends f.ex. 90% S&P500 ETF asset allocation, thouhg I feel MSCI ACWI ETC would result in even better risk spreading and decent longterm returns.

It depends on your investment criteria and your risk appetite. REITs are supposed to be less volatile in terms of price and are required to pay at least 90% of their income. Most SREITs distribute 100% of their income for tax reasons, giving them a much higher yield. However, if there is a need for cash, the REITs will have to go back to equity holders for cash.

Should investing in individual REITs be too risky for your choice, you can consider passive investing in REITs through REIT ETFs, which provide you with exposure to the REIT market, as well as allow you to invest in REITs that are diversified across industries and geographies.