Rainbow Six games have never had cutting edge graphics. Nobody should buy R6 for its graphics, but rather for the guns, gear, tactics, and teamwork. It is a blast with proper teamwork!
This is a high-realism FPS that strives for realistic graphics. Part of the reason Rainbow Six: Vegas was such a hit was that it had a dark, realistic graphical style that remained reasonably realistic in gameplay unlike CoD. The AI was always incredibly accurate and unpredictable which means a teammate and/or patience are necessary.
The R6 series has a relatively small fanbase with a pretty small budget provided from Ubisoft. This was supposed to be a sequel to R6: Vegas 2 (2008!) that was canceled in 2013. This is also the first modern R6 game that isn't on Unreal Engine. I think this game looks pretty good considering how this series was hanging on by a thread for the past 5 years.
It isn't fair to discount this game due to having dated graphics.