What's the risk attached with SME ipo's?

I've heard in most of the yt videos that SME ipo's are risky, you should be careful before investing in them but none of them ever explained what actually is the riskier part in it. I'm new to the market and all the recent SME ipo's gave a very good return and in most cases 90%. I want to know what is the risk attached to SME IPO's and also even if they're risky how come they give such good returns. Thanks in advance.