For every mod you add, complexity usually increases exponentially.

Depending on the game, difficulty also varies: modding stardew valley is joy (117 mods in a pack, easy afternoon sipping tea), modding skyrim less so (oh god,these two amazing mods tweak the same tree, time to go patch hunting, 2 weeks later you play it only to spot obscure graphical glitches, all hail wabbajack automation!), trying to make a working multiplayer mod pack for rimworld is pure suffering (why do you hate me, why do two compatible mods generate mass instability?!? 4 months of bug hunting and unsalvageable runs due to strange mod interactions, gave up for now).