I think the answer is mostly that it's hard to route most traffic over SOCKS, most applications don't obey the system-wide proxy settings, you might be leaking DNS queries, things like that.
The most compelling aspect of sshuttle, in my opinion, is that any host running sshd can be an endpoint - no software is required. As long as the sshd in question has python available, you can use it as an endpoint.
my point is: on all setups you have traffic that might go around the vpn tunnel, that is an application defect not that ssh tunels are worse.