So I have a server, and was wondering if structures are not generated if you add the mod later. I've been roaming around a tundra, and haven't seen anything. I added the mod not so long after we started it, and I'm not sure if is not working or we just haven't found a single one.

Try using /locate to see if it returns anything. If it does, it might be that the structures are just rare. (We don’t add much to tundras anyway)
Ill put this for 1.2.1 for rebalancing

yes, Structures (any worldgen) will not affect already loaded chunks because they've already been loaded. Try exploring out where you haven't gone yet, you should be able to find them.

Rereading this, yeah it doesn't regenerate where structures should be. Only during newly generated chunks.

Rereading this, yeah it doesn't regenerate where structures should be. Only during newly generated chunks.

OK It does work, but the pillager factory I found almost didn't generate pillager. I only found 2 pillagers inside, and I don't know if it's not supposed to happen.

That's just minecraft being weird, i can't help with that. youre fine.