this may be late, or maybe you already know more than me by now ( nearly 2 months later )
using a custom buffer size can be beneficial, but only going down not up. Larger buffer size gives your stream more variance with upload and thus better quality. The issues will crop up if you do not have transcoding on a stream.
For example: If I am streaming with a 5k bitrate and I try to use a 5k buffer then that will allow OBS to increase or decrease by that buffer number. So if I am playing a fast paced FPS game then OBS will actually go above my 5k limit to keep quality higher. It uses the buffer as "permission" so it thinks it can go above the bitrate by the number of the buffer. On the other hand if I am streaming something very slow paced with little to no movement ( face cam only video ) then OBS will keep the bitrate lower because you don't need high bitrate.
I hope that makes sense. The issues you will run into are people watching the stream. If there is no transcoding, such as on Twitch, then they will have more issues with a higher buffer vs lower buffer.
Transcoding lets the viewer pick their resolution ( 1080p or 720p or 480p etc ) so someone with a slower internet connection can select a lower resolution and watch with little to no buffering. Most platforms have transcoding, so people can choose what resolution they want to watch.
Twitch specifically will only use transcoding for preferred streamers, so not everyone will have transcoding on their streams. So, with Twitch specifically and maybe other platforms, you want your stream to have little to no change in bitrate. When your bitrate goes up and down it causes buffering because your viewers have to "catch up" to your bitrate.
So, long detailed explanation short, a higher buffer will make your stream look better, but at the cost of more loading / buffering on the viewers screens. A smaller buffer is more stable / less loading and buffering for viewers but you will lose some quality.
This will not matter if your bitrate is already good enough. So streaming a 1080p video with a 20k bitrate will look amazing with no buffer at all, but will be crazy hard to do depending on your internet speeds, stability, etc. Buffers are great, but most people don't need to use a custom buffer. By default OBS will have a 1 second buffer built in, you can adjust it lower for better stability at potentially lower quality. Or you can increase it for worse stability but better quality. Streaming only of course, recording video locally is totally different.
