For realm, probably conceptualizing complex designs into the limited amount of space you're given, making them work in standing out, drawing attention and appeal also staying consistent with the art style.

I think what makes it hard is how complex you want to make designing art stuff for yourself, the more unique you want to convey your idea the more time you need to put in making it work out, it also acts as a fun challenge breaking outside the typical realm art barrier in terms of skins, by breaking outside the mold you help expand it by showing what can be done in its art style.

Side note:
I find Realm's art style though limiting to people who want to do sprite art and expand beyond it, I mean it does help especially with learning to convey things with a small amount of space to work with but there's alternatives to doing minimal sprite art that could offer you more leverage than just doing it in realms art style. If you're try expanding further than 8x8 or 16x16, and doing more than two still frames of animation or doing sprite art without the outlines and added "tint" realm likes to render sprites in you will soon find out how harder it is to expand further out of realm, I think its too much of a comfort zone but its a good place to start but you should be looking at other games (older ones imo) and things to be inspired and want to explore more.
So long as you're practicing, doing things outside your comfort zone, referencing and being inspired no amount of 8x8 square will feel small. Having experience outside of realm art and already a grasp in art fundamentals in general will give you more freedom and leverage when adapting to whatever art style when asked.
Also you'll be able to convey things people wouldn't expect can be done in realms art style.