Are you currently (or with the newer version) able to tag someone in discussions in SE?
In 4.10 we have username tagging (mentions) .
I migrated my author site to SE. If you are still a member, feel free to log in and explore. I had no trouble. I use Groups and it has Discussions (forum).
I use the Atlas theme which is responsive so I don't need mobile app and don't have those fees associated with mobile apps. There are mobile apps available.
I love the control I have in SE, networks are awesome - I use them for genres. With 4.10 coming soon, we'll have hashtags, smileys and more control over networks and profile mapping so it'll be pretty complete.
I don't miss much. My ST main site is PF v3. If you were to use PF, I suggest v3 over v4. Less issues. However, SE has a better setup for CDN and external storage. Honestly, you should ask @Zixia604 as he just migrated a huge community. Perhaps PM him.