Well, one of my DDs (now an adult) broke her arm when she jumped off the upper bed. She was 4 or 5 at the time. The other downside is that they are hard to make.
My DD has a loft bed that looks like a house.
Tradewins Doll House Loft Bunk Bed
It DOES take up a lot of room but it is so enclosed on the top that (1) I never see her messy bed and (2) it is harder to jump off (harder, not impossible I have learned). And it did free up a lot of room for storage underneath.
If you consider a bunk bed, I highly recommend some kind of canopy cover over the top of the upper bed.
Knight's Castle Loft Bed
A canopy takes up more visual space but deters jumping. Sadly, it also deters adults from going up there and reading or snuggling with their child. Plus it is A LOT harder to carry a sleeping child to bed and put them in an upper bunk, especially with very tall bunk beds. The drawback to all bunk beds and loft beds, in my opinion, is those darn stairs and ladders. If it is a minimal ladder, it is difficult for adults to get up. If it is a stair system, it takes up more room.
Lastly, remember that heat rises. It is always hotter in the upper bunk - good during the winter, not so great during the summer.
Whatever you decide, keep in mind that a bunk bed or loft bed takes up A LOT of visual space. They can quickly overwhelm a room. DD's loft bed barely misses her ceiling fan. Once you narrow your choices down to a few options, use the dimensions to map out exactly where it will go in the room. Will it block windows? Lights? Doors?
Oh, and I'll mention one more thing ... with young kids all sorts of things used to fall behind the bunk bed - which is NOT a piece of furniture that is easily moved so you can vacuum under it. I was forever digging out tiny toys and broken crayons from behind the mattresses. As a matter of fact, with DD's loft bed. It is nearly impossible to extract the mattress so if something falls under it, I practically have to be a contortionist to remove it.