When we tested Instagram, our post went through without any issues 🎉 When we tried to use Metricool for Instagram, we had major issues with the video file size, and the Metricool team said we'd need to re-size every video in order to post them through Metricool.
We had none of those issues with Buffer which was a HUGE win! Posting to Instagram was reliable and stable. You can format the captions, store hashtags, and choose a thumbnail.
Our content doesn't get a ton of views on Instagram (most of our audience is on YouTube, given we are a B2B channel), but we figure we might as well re-purpose the content for Instagram while we have it.
While we did see a slight decrease in views when posting via Buffer instead of Instagram directly, it wasn't anything major, and in general our content never gets a large number of views on Instagram. Because Instagram isn't a platform we care about much, we figured the time-saving from using Buffer to post outweighed any cons.
If you are thinking of using Buffer for posting to Instagram, I'd keep track of your video views and see if it's impacted at all from when you were posting manually.