The space is actually very harsh on living things . They probably synthesis their own air supply , so you couldn't just have infinite air supply outside Ishimura . Of course in reality , there shouldn't be just 70 seconds of air supply , but as the Necromorph overrun the whole Ishimura , everything became scarce . So instead of maybe few hours of oxygen to work with for the previous engineer , Isaac only had few minutes .

For other weapons as well , the idea of scarce items at Ishimura forced Isaac to have very limited ammo or materials to synthesis ammo .

About the USG Valor , yeah Ive thought about this too . According to Hammond , it supposed to be a war ship , however easily defeated . In my guess , the Valor just didn't have the idea of what they are actually facing . Besides that , they are being caught off guard , because they thought the pod was friendly .

My idea of how this actually happen is the pod contains a walking necromorph and few babies Necro. As the Valor's soldier investigate the pod , they found an unknown being charging toward them . Not knowing how to deal with it , they shoot the body which obviously doesn't do any damage . The Necro successfully killed one of the soldier , while the others are being picked by the babies Necro . Soon , the Necros are multiplied and hunt for the rest of the crew . Necros are pretty smart , they don't fight faced on , instead they go through backdoor and vents , and so this might be how the Valor was overrun .

It doesn't matter how harsh the space is, air is air. We know the suit draws air in from the enviornment to fill up it's own internal air supply. We know that the air in the ship is breathable or else Isaac be dead a long time already, so the same question applies, who makes a space suit with only seconds worth of air.

- About the USG Valor , yeah Ive thought about this too . According to Hammond , it supposed to be a war ship , however easily defeated . In my guess , the Valor just didn't have the idea of what they are actually facing . Besides that , they are being caught off guard , because they thought the pod was friendly .

Pay attention to what Kendra said, the Valor was HER backup ship. Which means the ship was meant to find and retreive back the marker. But you're still right... the people on the ship didn't have any nail guns, or cutter knives so they died when their guns were ineffective.

You'll know how easy Necromorph can overrun a ship , if their weakness aren't discovered .

About the suit , you might be right . Perhaps this is just the gameplay elements .

The main problem with the Valor is that they didn't have any NAIL GUNs on board!

Unless the pulse rifle, contact gun, force gun, flamethrower etc are all prototypes, it doesn't make sense if their problem is ineffective weapon or tactics. Valor is a military ship on search'n'destroy mission, with trained military personnel and big ammunition, there's no way necromorph can take over the ship in such a short time without an army of them.