So I understand a lot of world builders and aspiring fiction writers have often tried to be creative with weapons and get shutdown for various reasons of impracticality or inferiority to steel. I'm thinking why not play into that? Why not have all those weaknesses be part of a closely guarded secret known by few not unlike the Pentagon with military secrets? In fact, I'm sure there's plenty of real life military equipment specs that are closely guarded not just to avoid adversaries copying it, but to avoid having its crippling weaknesses discovered by the enemy.
So with that in mind, how feasible would it be to have an elite guard equipped with magnesium alloy swords that can be ignited into flaming sword either through specially created gauntlet or sheathes with embedded flint edges? I certainly expect it to be a terrifying sight to see.
Would its weaknesses be immediately seen by a trained fighter in combat through a simple clashing of blades? Even if so, I don't expect there be a reason for unarmed peasants to be able to tell its weakness while being singed and slashed at?
How would they most effectively employ the weapon and guard its secret? How often could they employ such a weapon?
If it is simply not an effective practical weapon, is there a way at least to use it to "bluff" the enemy to believe that it is?
EDIT: Thanks for all the responses. Some of the comments were very enlightening to me and has been helpful in revising my idea. There's still some that I see my preface to the question didn't help steer in the direction I wanted.
For example, I don't see the fact the sword burns away as a crippling detriment, so long as it remains functionally useful and retains its form for a few minutes until the encounter is over - the enemy doesn't need to know that; this elite group may only deploy the weapon when they know the encounter will be over within a few minutes. The fact it won't survive blade contact is also not an insurmountable problem; they would use a sword style that emphasizes on dodging rather than parrying. This leaves something for protagonists to observe and eventually deduce its weakness and limitations.
As for function, well, striking fear and confusion into the enemy is already a great advantage, as is damaging and melting the enemy's equipment. They might advertise themselves as the god's chosen enforcers or some such and cleanse their enemies with divine fire when its really just science.
The fact that it would be just as dangerous if not more so to the wielder is a good point, but that need not be an insurmountable issue as the wielder can prepare themselves with proper equipment/usage as opposed to their victims.