FortiMail is a very flexible solution easy to install and use, any organization can use it whenever they require a virtual, physical, or cloud instance. Also, it is great to have an email archiver out of the box, you do not need any external product for it. The only thing that organizations need to consider before purchasing is service bundles - with the additional cost you can get Content Disarm and Reconstruction, URL click protection, Cloud Sandboxing, etc. (included in Enterprise Advanced Threat Protection Bundle).
It is very well suited for an organization that wants to protect inbound email and use just one product. It may be less appropriate for an organization that is small or has a limited IT support staff.
Any time I've had to use the support, it has been great! They always will recommend certain options, as well as explain what they do and why they are there. The support always gets back to me the same day, and are always pleasant to deal with. I never hesitate to call.
I have used Barracuda Email Security in the past and FortiMail is just as effective. The interface is easy to use, and the configuration is straightforward in setting it up to work with Microsoft Exchange. Spam detection is effective, as well as malware/viruses and phishing attempts. Overall it is a great service.
It would be difficult to determine the exact ROI Symantec Messaging Gateway had on your overall business. However, I can assure you the downtime suffered due to inbound email threats has been reduced to nearly zero in our environment. If IT isn't chasing down ransomware/phishing issues and users aren't staring at a quarantined PC, life is good.
There are no negative impacts on ROI that I can think of.