You can’t draw a full length sword from your back. You’ll occasionally see people screwing around, trying to find a way to make it work, but it’s not something you want to be messing around with in combat.
Pulling a sword from your back, involves juggling the blade a bit while drawing. Again, if you’re devoting your full attention to this, it’s completely safe. If someone is already invested in trying to murder you, you cannot devote your full attention to pulling your weapon, and the entire situation becomes much dicier.
Swords on the back can be useful when you’re traveling, and not expecting combat, since it gets the weapon out of your way, completely.
Knives and pistols are sometimes sheathed/holstered horizontally at the small of the back. It’s a good place to conceal one, since the curve of the spine will hide the weapon’s outline under loose clothing.
You’ll sometimes see people sticking machetes on their back or across their waste. As I’ve said many times before, these aren’t actually weapons, they’re tools, and both are convenient places to get them out of your way while still having access to them.
As a general rule, anything longer than your arm, can’t be pulled from a back sheath or holster. There’s some exceptions to this, like modern pressure sheathes, which will hold a weapon in place while leaving the sides accessible, so you can change the angle while drawing the weapon.
Even failing that, drawing from over the shoulder leaves your body exposed to an incoming strike, in a way that belt sheathes do not. Even if you’re wearing armor, the underarm is one of the lightest locations. In most cases with plate armor, there will actually be a gap. This is because you need to maintain freedom of movement in the shoulder, and you can’t do that if your arm is completely plated over with solid metal. So, raising your arm to draw from your back actually exposes that weak point to your foe.
But, if you’re expecting to be on the road for the next few weeks, and you’ve got a walking staff, then sticking a sword on your back is fairly reasonable. It won’t be bouncing off your leg, annoying you, the entire time. Just don’t expect to be able to get to it if you’re ambushed by bandits.