It's possible to place furniture outside, but only if the house has an instanced courtyard. Not all of them do, but you can check when you preview the house before buying.

Placing it on uneven terrain can be tricky, but it's possible to make most items look ok. The biggest problem is usually that you can't use the right-click to align option, you have to rotate it manually until it looks right.

I found the biggest problem was trying to decide what looked right. Sometimes things look better angled so they're lying on top of the ground, sometimes they look better upright but clipping into the ground slightly and sometimes they look better raised very slightly above the ground so they don't clip. It depends on what it is and where you're putting it, but in practice it's not too difficult to sort out.

The Khajiit houses from medium and up has yards but if you could not use it and the yard are around Serenity Falls has lots of uneven terrain who would be pretty pointless if it could not be used
So thank you, it makes sense too.