Are there any US stocks restrictions for Singaporeans to invest?

Usually restrictions are set by the brokers. But in terms of general access to publicly listed US stocks, as long as your broker has access to the US market, you shouldn't face any restrictions.

Certain brokers will implement restrictions that prevent the purchase of:

1. Leveraged products (TQQQ, UPRO)

2. OTC/Penny stocks (TCEHY, MMDEF)

3. Products that utilise derivatives (UVXY, USO)

For some of these brokers (like Interactive Brokers), you can apply for permissions to allow these stocks. But for others (like SC), you might not. So you'll have to check with your broker's customer service!