Depending on who your customers are, where they are, and the types of devices they use (PCs vs Mobile), the results can vary.
In practice (based on my experience), ICE connectivity with STUN alone is about 85% successful for desktop and laptop PCs. But if it works once for a particular pair of endpoints, it will be even more likely for subsequent connections for these same hosts (assuming network topologies have not changed). Things are a bit different for mobile devices.
Here are some factors that influence getting a successful "connection" (either UDP based or TCP) for P2P.
NAT type. If both endpoints are behind well behaved "port restricted" NATs or better, then chances are high for success with STUN. This is the usual case for home NATs with a good ISP such as those in the US. But mobile carriers and enterprise firewalls typically implement "symmetric NAT" as a result of having multiple layers of NATs and network configurations. This basically means that port mappings are not consistent - and are harder for a P2P algorithm like ICE to establish a connection with.
Firewalls or Enterprise configurations. Even if the firewall allows outbound UDP packets and accepts packets back, it is often a symmetric NAT configuration.
Mobile carriers. Are often (but not always) symmetric NAT types.