A lot of people don't realize that all these so-called issues with TCP, like slow-start, Nagle, window sizes and congestion algorithms, are not there because TCP was badly designed, but rather that these are inherent problems you get when you want to create any reliable stream protocol on top of an unreliable datagram one. The advantage of QUIC is that it can multiplex multiple reliable streams while using only a single congestion window, which is a bit more optimal than having multiple TCP sockets.
One other advantage of QUIC is that you avoid some latency from the three-way handshake that is used in almost any TCP implementation. Although technically you can already send data in the first SYN packet, the three-way handshake is necessary to avoid confusion in some edge cases (like a previous TCP connection using the same source and destination ports).
They also tend to focus on bandwidth and underestimate the impact of latency :)
Interesting to hear that QUIC does away with the 3WHS - it always catches people by surprise that it takes at least 4 x latency to get some data on a new TCP connection. :)
One other advantage of QUIC is that you avoid some latency from the three-way handshake that is used in almost any TCP implementation. Although technically you can already send data in the first SYN packet, the three-way handshake is necessary to avoid confusion in some edge cases (like a previous TCP connection using the same source and destination ports).