Actually I have made some progress overriding/modifying themes in PFv4. It's taken me some time to get used to the new layout, but so far good.
I found a fix (sort of) for thumbnail cropping problem in the Material Theme. Using the Inspect tool in my browser (I use Chrome) I identified where in the bootstrap.css file the code is for each of the photo thumbnail containers (i.e., Album Covers, Photos, Photos in the activity feed, etc.). I then changed the background-size from "center" to "contain" and I added a background-color, in this case black (#000000). It's not the perfect solution, but it looks much much better then have heads cut off.
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.