The only time i experienced an issue with invisible models was when i included custom ammo boxes into a map file.
During map transition from a map with regular ammo boxes to a map with custom ones or vice verse, they were invisible.
I don't know exactly, but i guess during map transition, a few things like models that are always used are keept in memory to save loading time.
Now if a change occures on this model, the engine cannot fully load it due to conflicts with the allready precached one.
Custom textures on the other hand will work even if they're included into the bsp.
While adding custom characters to your client is easy in GES, the difficulties come with online play.
(note that i'm not talking about model sweps here)
If you'd add a brand new character to your client, let's say Xenia, you might wanna use her online.
But other clients and even the server don't have the neccessary files to display the model.
Even if a server would distribute the model files via download:
1. there are certain files that cannot be modified by a server download
2. the precaching of scripts and gui elements might cause issues too,
at the very first connection for sure.
3. what if there's a server running without the custom models?
I'm not a programmer, so i don't know if there might be a solution for all these things.
Then again, it's not only a question of time and effort that needs to be put into that,
but also a question of the benefits vs the risk.
Let's say there would be a system that allows the user to add their custom characters to the game in a way that everyone is able to see it on his client too.
I bet we'd soon see a lot of stuff unsuitable for the mod but only a few characters that are really worth adding, if any at all.
We're not GMOD, so anyone who wants to see giant walking penis and nude Alyx, should go there.
If anyone has a character that's really worth adding it to the mod, it can be also posted here. Just like DrDean did.
(the guard model ofc. since the others are property of EA)