I wasn't referring to tactics just the brute strength of their abilities compared to security forces, armed civilians, and medium infantry with a couple of years of training. In the Novaria situation they pop up almost directly on top of the security detail and are still gunned down before they can close the 10 feet to their targets? As for the marines, they do have some cover but many of the Rachni never get within spiting range. Those comments were more towards previous statements about their natural advantages in combat over Krogan when a few armed infantry can hold back the same amount of pure Rachni and Krogan are reputed to be better then humans.
Yes its true that the Rachni are gunned down before they can get close, but that is to be expected when you charge in a straight line at an entrenched force with automatic weaponry. The same scenario plays itself out when a Krogan Battlemaster charges Shepard and Co. in Mass Effect 1 as well. There are many times when the player is able to down the Krogan combat veteran before he even gets close to engaging in melee; even on the insanity difficulty. Both species are going to get shot up if they blindly charge a well equipped squad of trained combat personal, so I am not sure how the Rachni would be inferior in such a situation.
At least the Rachni the player encounters in Mass Effect 1 have a reasonable explanation for zerg rushing people, they are newly born soldiers that have been driven mad by lack of contact to their queen. They are operating on base instinct, and like a cornered animal they lash out. What excuse does a hundred(+) year old veteran of combat and master of battlefield strategy and tactics have for sprinting directly towards military personal with long ranged weaponry?
I do acknowledge that tactics and position can turn any battle and the Rachni seem to have mastered ambushes till they become instinct but unless you feel like admitting that the Krogan are so talented at battlefield strategy that they used an inferior force to take out the Rachni on their own turf dozens of times over it's more proof that the Krogan had the brute strength and numbers to defeat the Rachni at their height. This has to be factored into the decision of the in game characters as well, when your preparing for a fistfight do you bring along an undefeated champ or someone who's been knocked out a half a dozen times, sure they get back up for the next fight but that doesn't change the fact that they lost.
The Krogan helped turn the tide of the Rachni Wars, but it's not because they overpowered their opponents in open combat; the entirety of the Krogan army didn't face the entirety of the Rachni army and win. The Codex specifically mentions that the Krogan were able to strike the Rachni hives and attack the queens directly, but this seems to be talking more about small surgical strikes vs. open warfare. I am not dismissing the Krogan's accomplishments during the conflict, but a commando insertion behind enemy lines to assassinate a target doesn't automatically make the defending force militarily inadequate.
It is also important to note that there were other races fighting during the Rachni wars than just the Krogan. They might have pulled the trigger, or planted the bombs, but I highly doubt it was them that did the planning. From everything we are shown or read about the Krogan in the setting they don't seem to be ones for subtlety or discretion. More than likely it was the Salarians that drew up the plans, that plotted out the insertion points into the nests, etc. The Krogan's resilience and high birth rate allowed them to weather the hostile nature of the Rachni nests and compensate for a high casualty rates, but no mention is ever made about the keen intellect or battlefield prowess that they brought to the fight. Understand me, I am not calling the Krogan stupid brutes, but the Salarians uplifted them for one specific purpose.
To use the fighting/boxing metaphor you gave: Its not so much that the Rachni were beaten single-handedly by the Krogan, the undefeated heavy weight champion of the world, in a knock down drag out fight for supremacy. It was more like the Rachni were holding their own against the tag team of the Asari and Salarians, and the Krogan came in and sucker punched them from behind (IMO).
The Rachni do seem to be a more peaceful option but there is 1 catch to the whole thing. Trust. It requires the absolute belief that the queen is telling the truth. Sure, she seems genuine in giving you the choice to let her die. But she doesn't really have a choice in the matter either time. Appealing to Shepard's humanity that she is no threat is the only way to save herself and her future children. There was never any diplomacy during the Rachni wars because they killed the council's delegations so we don't know if they are a trustworthy species or not. At least with the Krogan you know what you get, the devil you know as they say.
That is true, but their are two points that go contrary to that assumption.
1) If the Queen was just playing Shepard so that she could escape Noveria, then why would she send a messenger to him/her on Illum? Even if we assume that the Rachni was mind controlling the Asari in order to procure supplies why would the Queen give the Asari a message to relay to Shepard and actively seek him/her out, drawing attention to her presence? If her declaration on Peak 15 was just a ploy so that she could saver her own exoskeleton, why would she bother letting Shepard know that she was still around and operating in Citadel space?
The Queen could have just as easily fled known space and gone into hiding, leaving Shepard and the galaxy high and dry but she didn't. Even more than that, she specifically sought Shepard out, and let him/her know that she was building an army to help in the upcoming battle with the Reapers.
2) When Shepard rescues the Queen in Mass Effect 3 nothing is stopping her from laying low and discreetly trying to fade into the background while the galaxy focuses on the Reaper invasion. Sure, she promises aid, but its not like Shepard placed her under armed guard when he/she freed the Queen from Reaper captivity. The Rachni could have hijacked a transport, and FTL-ed into uncharted space. There was plenty of confusion and chaos happening all over the Krogan DMZ during that time, a lone freighter going missing would have been chalked up to Reaper attack, or desperate pirates. Even if the allied forces of the galaxy would have known that the Queen just ran away while giving everyone the Rachni equivalent of the middle finger they would have had neither the time or the resources to track her down, nor would they have shifted their attention away from stopping the Reapers.
The fact that the Queen not only stays with the allied forces, increasing the chances that she would be rediscovered by the invading Reapers, but sends aid to the engineers building the Crucible proves that the Rachni are committed to the war effort, even though it means that they will be joining the rest of the galaxy in extinction if they fail to stop them.