To me, there is TWO things that are underrated when upgrading components are you bike. Modern dirt bikes are really good in stock form. Most people don’t need to mod their engine or doing anything wild.
My number 1 underrated bike upgrade:
Getting your suspension sprung for your weight. I am not talking about valving. Just simply getting the springs for your weight for the bike you ride. A few factors go into determining this; your weight, your height, your skill, and what kind of riding you do. Be honest on all of this to get WHAT YOU NEED. After you have spent some time on the bike with the correct springs, we can talk about valving then.
My other number 1 underrated bike upgrade:
Handlebars. I feel like this is more of a factor for the shorties or the tall ones. When I get on most new bikes, right away I notice the bars. They feel like ape hangers and would definitely not be ideal to use. Having bars that are not good for you, can put you in the wrong position when riding and might even make your hands / wrist hurt. My bar of choice is Fasst Company Flexx Bars 14 degree, enduro low.