Lemme see if I can parse this out...
I've hit heavy bags barehanded, with bag gloves only,and with wraps and training gloves.
The main danger in hitting a bag barehanded is that you'll hit the bag at a slight angle and your wrist will collapse, causing a wrist sprain. Same thing can happen if you hit the bag squarely and aren't holding your wrist tight. You can minimize the risk by starting off lightly and working up to harder punching. The bag will also abrade (scrape) the skin on your knuckles, but that's a minor issue unless you spend a lot of time hitting the bag right off the bat -- eventually, you'll develop calluses.
Hitting a bag with bag gloves only poses about the same risk of sprain. Plus, if your hands sweat a lot, the sweat can soften up the skin on your knuckles, which can lead to tearing your skin. I used to wail on a heavy bag with bag gloves, and about once a month I'd lose a dime-sized chunk of skin off my middle knuckle. Hurt like hell,and I had to stop punching with that hand until the skin grew back.
Using wrist wraps and training gloves is the safest option, but it's not an absolute guarantee you won't sprain your wrist. A hard, badly aimed shot can still do some damage.
I'm sure that wrist wraps + bag gloves would be about the same, but I've never used that option.