Not every function on the Instant Pot is 100% perfect, and for me, that's been the case with the rice cooker. Rice perfectionists will tell you that the IP isn't a replacement for a rice cooker. The main complaint is that it's inconsistent. Something like barley takes about 25 to 30 minutes to cook and has a 1:4 grain to water ratio, while quinoa can take 8 minutes with a 1:2 ratio. Make sure you consult the guide to make sure you're cooking your grains correctly. I've heard that using the "Rice" function with white rice does work, though, so experiment for a while until you find what you like!
However, it goes a pretty good job (especially when making a rice recipe — not just plain rice) when using the proper times for the style of rice. It's not bad by any means, but you might want to consider the features that matter to you most when it comes to cooking your rice in the IP or a rice cooker. I actually have and use both, so that it's easy to make a recipe in the Instant Pot and have a cup or so of plain rice ready and waiting to go.