I've had an in ground shelter for about 8-9 years now. It is a two piece unit that is glued together in the center. I haven't had any leaks at the seam so far thankfully. When the ground is VERY wet the cement towards the bottom gets a bit darker like it has more water/moisture in it, but I've never had a leak or any standing water. I've got a small carpet in the bottom and it stays dry.
I find the debate about above ground and below ground units somewhat entertaining. It is probably a situation of underground being a 10 in terms of strength and the above ground being a 9. Logically I don't see how you can argue that an above ground is stronger as it doesn't have feet and feet of solid ground to protect it, but they certainly do seem strong enough to handle strong tornadoes. My advice is get what you can, or what works for your family. Some families may have people with limited mobility and getting down into an underground shelter may not be an option.
I find the debate about above ground and below ground units somewhat entertaining. It is probably a situation of underground being a 10 in terms of strength and the above ground being a 9. Logically I don't see how you can argue that an above ground is stronger as it doesn't have feet and feet of solid ground to protect it, but they certainly do seem strong enough to handle strong tornadoes. My advice is get what you can, or what works for your family. Some families may have people with limited mobility and getting down into an underground shelter may not be an option.