I have been streaming at 1080p 60FPS at around 6kbps with no artifacting at all for about 6 months. The difference in my case is that my base resolution is 1080p. i would check your scale sampling rate under settings > Video > Downscale filter. Also, if you ever have a problem with OBS and post in the forum it is a good idea to include a log file in case you are having hardware issues.
Sorry, but "no artifacting at all" with 6000kbit/s CBR @1080p 60fps is not possible in games like PUBG, Overwatch, Rocket League in fast motion scenes (at least not with x264 very fast, faster or fast).
You can record/stream Rocket League in high details at 1080p 60fps with 6kbps and see, that there will be some pixelation in some scenes, which is totally normal.
1080p 60fps is a bad choice for many fast games (as long as you can not use >10.000kbit/s).
On the other hand there are games like League Of Legends or Hearthstone that will look very decent in 1080p 60fps, as they need less bitrate.
You can make your footage more encoding-friendly, by using bilinear scale filter instead of lanczos filter or by reducing ingame details or by enabling motion blur (if the game supports it).
Everything that reduces picture details in motion, will make the material more encoding friendly.
https://www.youtube.com/watch?v=r6Rp-uo6HmI