The Covenant have become known for their almost laziness when it comes to technological hardware - it’s been discussed before, the Huragok being the most important feature of the Empire, the San 'Shyuum being a joke, etc etc.
But the question is - how imitative are they?
Sure, numerous aspects of the Covenant and it’s technology are derived the Forerunners. Hard-light, holography, and so on, but many things just do not match up. If we assume the Huragok who work for the Covenant are the same who worked with the Forerunners, who created weapons, ship, and so on with the utmost capability, why is it so many things the Covenant use are different? Plasma weaponry, overall differently designed weapons and ships, weaker energy shielding, and a degree of actual incompetence in many manners of warfare, with almost minimal stealth, espionage, or software superiority compared to humans.
Would the Huragok really design big purple bulbous ships? Or did the San 'Shyuum suggest this? Why would they? Despite what is said in many accounts in the Halo Universe, it does seem that, while the Huragok are the engineers, a lot of the thinking goes to the San 'Shyuum.
For one, the designs of Forerunner ships to Covenant ships have always been drastically different. Forerunner ships were larger, less streamlined, and more boxy, while Covenant ships are generally average in size compared to the Forerunners, extremely bulbous and smooth, and very bulbous. And, though this could be attributed to materials, the colouration is bizarrely different. Surely the Huragok don’t take aesthetics into account.
And then the matter of plasma weaponry must be raised. Why would the Huragok create plasma based weaponry when they could simply produce the same laser weaponry they did for the Forerunners? There is little reasoning for this, as plasma is less effective, less accurate, and more difficult to attain than that of lasers.
While it is evident the Covenant imitated many things from the Forerunners, I do not believe it is entirely fair to say they’ve never innovated. The (possible) choice to use plasma over lasers is an already notable example of this innovation - not smart innovation, but innovation none the less.