STO is Secure Token Offering that is mean this token is really secure for investor, but to lauch STO is need big money and high level condition to uprove it.
That's why STO is hard to lauch but good to invest.
Apart from this the main handicap at the moment for security tokens is the lack of liquidity. There are only a bunch of countries where an STO can be launched, very few regulated exchanges for STOs, even less security-law compliant blockchains and hardly any users familiar with these and registered in the aforementioned exchanges. Of course this will change with time but at the moment this is lagging the market.
And in general how good an investment is doesn't depend so much on whether the company is launching an ICO, STO or IEO, but the strength of the idea, the team behind it and the tokenomics (this last part is way too often disregarded and it plays a key role).