Simplicity comes to my mind

1. Low cost
2. Connects 99% of time
3. Easy to use
4. Developer Friendly
5. Could be a little more secure but its secure enough
6. Easy access to Tor network
7.simple , download, hit app and your in motion
8. I was not forced to write review and Im not aware of a pro version but if there is I hope the developer lets me know

CappyCatopolous about Red Onion - Darknet Browser, v2.2