Back in 2013, United pulled Singapore awards from the United website, so that you could still book United awards but you would have to call in to make those bookings (which wasn’t always a smooth process). However, as of today, Singapore Airlines awards have returned to United’s website making it much easier to book these awards.
The big caveat is that the same booking policies apply that limit Singapore’s award inventory available to Star Alliance partners. For example, on many long haul flights and those served by the A380 it has been virtually impossible to use Star Alliance partners like United or Aeroplan to book Singapore business class or first class seats.
This is still in effect but you can find premium awards on routes served by the 777, such as the SIN -> SYD route below.
Overall, having Singapore awards show up on United’s website is nice but it’s not that exciting given the limitations imposed for premium cabins. Personally, I’m still not crazy about the fact that you can’t book many premium awards with Star Alliance partners but I suppose that it does work out for many of us. Singapore award availability would be much tougher to come by if it were open to all Star Alliance partners. I’m not sure how much worse availability would be considering how easy it is to earn Singapore miles via credit cards (Chase, Amex, and Citi) but I’m sure it would be worse.