RNG is a major issue as many know. Since everything is decided upon by random chance or a drop generator this can lead to unfair moments. A great example of RNG is during warframe's first nightwave event where you had to fight the wolf of saturn six. The drop rates for his weapon were so abysmal that DE made an event focusing around getting all of the items. It took me 75 runs of the hardcore version of the mission to get the part I needed to make the hammer, and thus it stirred the "RNG BAD" pot. RNG is also an issue in loot boxes where you don't know if you're going to get that skin you really want or just another common one.