Did your instances have external IPs? We had connectivity issues when using NAT due to it dropping SYN packets. We just gave everything that needed to talk to external IPs their own external IPs.
Anyway, yes. It caused us problems. Attempts to connect to machines outside of GCP would fail repeatedly due to the NA dropping the SYN. This would mean our connect() call would timeout repeatedly. There was an ICMP response I could see with tcpdump:
ICMP host aaa.bbb.ccc.ddd unreachable - admin prohibited filter, length 68