To make my point of this being an enormous restriction, let me just list out the top 30 most endorsed - so the highest quality voted by users - mods on Nexus and which ones will show up.
SkyUI: No, needs SKSE and external assets
A Quality World Map: No, needs external assets
Immersive Armors: probably not, uses internal assets to create "new" assets and likely won't be allowed
Unofficial Skyrim Patch: not sure
Appachii SkyHair: No, needs external assets
Skyrim HD 2k: No, needs external assets
Immersive Weapons: No, needs external assets
Enhnced Blood Texture: No, needs external assets
RaceMenu: No
Climates of Tamriel: No, uses external assets
Cloaks of Skyrim: No, uses external assets
Unofficial Dawnguard Patch: not sure
Alternate Start: maybe
Unofficial Dragonborn Patch: not sure
Static Mesh Improvement: no, uses external assets
Unofficial Hearthfire Patch: not sure
Realistic Ragdolls: no, uses external assets
Skyrim Flora Overhaul: no, uses external assets
Enhanced Lights and FX: no, uses external assets
Ultimate Follower Overhaul: no, uses SKSE
Wet and Cold: probably not, I think it uses external assets and some features need SKSE, but a limited version might show up
Realvission ENB: nope, and that goes for all ENBs
Frostfall: nope, uses SKSE and SkyUI
Convenient Horses: nope, uses SKSE
Water and Terrain Enhancement Redux: nope, uses external assets
The Eyes of Beauty: nope, uses external assets
Falskaar: no, uses external assets
Apocalypse - Magic of Skyrim: Yes
Immersive HUD: probably not
Amazing Follower Tweaks: Yes
So of the top 30, exactly 2 are guaranteed to be possible. Two out of thirty. And some of these are massive misses. Skyrim Flora Overhaul greatly enhances flora variance while enhancing rather than conflicting with Skyrim's art direction. Falskaar is an expansion sized mod with new land, NPCs, voice acting, the works and is excellently crafted. Oh and that's another thing: if sound files are blocked, you can't even do quests that need voice acting!
But yeah, the file size restrictions and lack of SKSE were bad enough, but with external assets blocked you lose out on the best work.
