Note the subscription licensing process, rules, benefits and how the third-party module activation process works can vary from region to region so note my experience is based on processes in the Australia/NZ/Oceania region.

Lots of pros:

1. More modules and features for the client
2. More regular upgrades - clients seem to now avoid those crazy long times between upgrades that turn upgrades into major events (like waiting 7 years until the system is too old before trying to upgrade)
3. Much faster and easier purchasing and licensing of new modules
4. Much easier licensing process - you don't have to re-enter activation codes each time you upgrade, just refreshes each year (well in our region we are issued the "88A" subscription licenses that work with any subscription-supported version.
5. Clients are much more likely to always be on a supported version of Sage 300 making it much easier to request assistance from Sage.

Only a few issues during the migration:

1. Tell all your third-party module providers you're moving to subscription to make sure they get put on the same renewal date cycle and so that Sage can contact them and provide updated third party codes each year your renewal is paid.  Some third party module providers also need to know because the way they issue you activations may be different.
2. You may need to add outgoing proxy rules at high security sites to let Sage 300 access the Sage licensing server.
3. You may need to open outbound firewall ports.