We have a need to setup site to site vpn's with several different clients (could be up to 100). The client endpoints have several different models and vendors appliances and we won't be able to dictate any software/hardware on their end. Our cloud provider has suggested us using an OpenSwan Linux server to accomplish this on our end. We have started setting up the first couple of these and have not had great results and we are a little nervous in proceeding in this direction. Does anyone have any experience with this to make us feel this is a good direction?
I think you will struggle to find a single backend solution to service such a wide variety of clients. The range of possible VPN configurations is very wide and not all implementations of the various protocols involved are always 100% compatible with the standards.
I would suggest to publish a "suggested" or "preferred" VPN configuration or a small set of these, after you have been able to confirm through tests that these actually work across multiple devices.