There are minor issues:
-There's no mini-door
-There's no flag thingy
-There's a minor animation distortion when the sims try to use it, for a second or so.
I imagine this is because the thing isn't intended to be used ANYWAY, but:
-You can still donate money
-You can still get gems cut and such
-You can still recieve mail
-You can still shift-click on it and get all the same cheat debug options as the regular mailbox
Read the following. READ IT. e_e
HOW TO USE:
-Go to decorative/misc and buy one for 500 simoleons
-Turn on Move Objects, or you can't place it
-Once you place it you'll still have another in your hand (like when placing lights), just click another button or object to get rid of it.
-Delete the original mailbox. This can be done using something like Awesomemod's "Nuke" feature (At least I believe it is awesomemod, it may be a base game testingcheats function) and shift-clicking on the mailbox, selecting "nuke".
If you want the regular mailbox, just buy it, too, and nuke the WA one. Both cost 500 and can be found in Deco -> Misc or Outside -> Lawn Decor.
I have not noticed ANY ill effects from this, so I am assuming that it is safe (unlike in The Sims and The Sims 2, where the mailbox had major functionality and deleting it would basically ruin the whole universe and everything in it). Tested a family for a few days, donated a few times, got some gems cut, recieved and paid bills. Everything still seems to work as normal. Despite that, use at your own risk. I see no reason it should cause issue, but if you're afraid of it, just don't use it. I am considering it in-testing. I also do not recommend sharing lots with one of these, just for safety purposes. If you're designing a lot around the mailbox you have issues and are crazy anyway.
I've also noticed no issues with multiple mailboxes. You can only use one, but having more doesn't seem to hurt at all, so it's cool for apartments and stuff.
DO NOT DARE only install one of these mailboxes, because I will not be held responsible if you build some great house using the WA one and then want to use the normal one. e_e Install both so that you can be sure you can always get the normal one back, damnit!