A-hem. Your data on these Gundams is not entirely accurate. The Deathscythe is in no way similar to the Abyss Gundam. The Deathscythe was developed as a stealth tactical mobile suit, equipped with powerful ECM jamming equipment to fool enemy radar systems. Its motif was "Grim Reaper", as evident by its demonic appearance, and a scythe as its primary weapon. The Deathscythe Hell was only an improvded version of this mobile suit, which also featured an active cloaking device to disappear from sight completely!
The Abyss Gundam was a Second Stage Gundam developed by ZAFT. It is an amphibious assault mobile suit, designed with aquatic combat in mind. It is armed with many beam cannons, which are intended to be used to bring down enemy battleship fleets. To maneuver more easily in water, the Abyss Gundam can transform into a mobile armor mode. I have not seen much of the series, but I can't imagine this mode would prove useful in space. Space combat is the specialty of one of its counterparts, the Chaos Gundam. The Gaia Gundam, additionally, was designed with ground combat in mind.
These three units were designed to operate in the three environments. Land, sea, and space. Savior Gundam is also a part of this collection, with its specialty being aerial combat. The Impulse Gundam is simply a ZAFT version of the Earth Alliance's Strike Gundam, with the ability to equip different mission packs based on the battle conditions.