Do the new STS and CSTS options add a little input delay ("input buffer") to help with connectivity issues? Kind of like how v-sync adds display lag when it has to wait for the frame to be fully rendered before displaying it? Is that why it's better to stick with Legacy if you don't have connectivity issues?
All models use an input buffer. The new options handle maintaining it differently, similar to how Overwatch does it. (See Tim Ford's GDC talk around ~25minutes in for more context on their approach)
The Overwatch approach involves very very subtly slowing down or speeding up the client's game simulation to maintain an input buffer size that can handle variation in network traffic.