The most visible issue is where the client's default gateway goes. In a full tunnel, it moves to the far side of the tunnel. In the split tunnel, it stays local. The security risk of split tunneling is that the client is providing a bridging path for outside malicious traffic to leak across the tunnel, with no influence from the far end's firewall and IDS. The performance risk of full tunnels is that 3rd party outside traffic not terminating at the organization on the far side still has to take the tunnel, which can add latency, limit throughput, or increase packet loss. The best designs require balancing the network layout, uplink sizing, and security posture in concert.
-- Jim Leinweber, WI State Lab of Hygiene
