On the server I played on the spy crate always turned you into a enemy soldier. Only.
The targeting box always gave you away but depending on the map you could sneak in and nuke a building. It wasn't easy, you needed your team to assist you somehow, either by rushing in or covering the beacon by shooting at it from outside of the base. It was way more teamplay oriented.
In renx if you get lucky and get turned into a tech then you can get a building depending on the map but I think the notification kind makes the enemy expect it so it's kind of pointless altogether.
There was also a stealth crate that would give your character the ability to cloak like the sbh, it did not protect you from base defenses though.