When a --uri is provided, the URI connection timeout (usually zero) overrides the default connection timeout of 3 seconds:

// in setOptionsFromURI in common/option/options.go 
opts.Connection.Timeout = int(cs.ConnectTimeout / time.Millisecond)

This manifests as a tool hanging on connection if a topology is down or if a cluster requires TLS and the tool isn't in TLS mode.