I have recently come to the conclusion that the Covenant was a poorly led military. The reason I say this is the fact that they use land invasion prior to orbital bombardment.
My reasoning is such: If the eradication of the Human Race is the order of the day for the Covenant military, then they would use any and all tools to achieve this end without risking the lives of their troops. The best tactic would be to render orbital defenses useless and then glass the planet form orbit.
What we have seen are headlong infantry invasions that serve little military purpose, and then retreats to ships in orbit that subsequently blanket the area in plasma. The main exceptions are Sigma Octanus and Reach, These two had significant Foreunner artifacts that were priority targets for the Covenant.
Every other planetary conquest ended in the inevitable retreat and glassing.
Had a human commander been in charge, he or she would have eliminated any orbital threats, scanned the planet for Forerunner artifacts, and once he or she determined that there were none, commenced to glass the planets from orbit. Invasion makes little sense if you don’t need the resources.
And before any of you question whether the Covenant need the resources, they do, and they are just as available from a dead glassed planet as an alive one. And dead planets don’t shoot at you.
So my conclusion is that the Covenant military leaders, the Sangheili, are poor military leaders that got thousands of their own kind killed for a sense of honor.