I've always liked the look of the Legioss (although not as much as the VF-1) and Shinji Aramaki, the designer of the Legioss is my second favorite mecha designer after Kawamori.
However, it's clear that Aramaki designs his mecha to look cool, without thinking too much of the real-world practicality or workability of the design, unlike Kawamori.
My major issue with the Legioss is that it has no method of pitch control in Armo-fighter mode for atmospheric flight. Although neither the VF-1 or the Legioss have any horizontal tail surfaces or elevators, the VF-1's thrust vectoring feet move up and down to provide pitch control. However, the Legioss's feet can only move side to side and thus cannot provide any pitch control.
IIRC, we once had a discussion in another thread about whether the Legioss would fly in an atmosphere and the consensus was that it would not (David Hingtgen or other knowledgable MW aviation experts, please feel free to step in and correct me if I am wrong).
Other issues I have with the Legioss are: -
1) In Armo-fighter mode, the boxy shoulder missile launchers are very unaerodynamic and would probably create lots of buffeting and limit maximum speed in atmosphere to sub-sonic.
2) While the Legioss carries lots of micro-missiles, they are all short-range only. Lack of any medium or long range weapons is a major disadvantage.
3) While the curved gunpod magazine may look cool, it's a impractical as far as carriage of the weapon in fighter mode. The magazine actually just is a battery pack and does not hold cannon shells, so there is no reason to make it curved and have it sticking out at 90º to the gunpod body. Heck, on the toynami MPC you have to remove the magazine to attach the gunpod to the wing in fighter mode!
4) having the wings folding down in Armo-Diver mode provides less lift.
Graham