Unmodded Tenno-originated weapons provides balanced stats against everything, which destroy Grineer and Infested alike. Corpus knows this from studying Orokin technology that they came up with their counter : lots and lots of energy shields. No wonder Ospreys were so annoying.
Early-tier Grineer weapons tend to be effective against Corpus: they were fighting them for years before you showed up.
On the whole, the ugly Grineer weapons compare very favorably to the sleek Corpus weapons. Why? Because Corpus are a megacorp and so their weapons are designed by marketing execs for sale. The Grineer are soldiers, and so their weapons are designed by soldiers for combat.
Excalibur's Radial Blind. In Arthurian Legend, the first time Arthur's reign is challenged, he drew Excalibur and it blinded his enemies, sound familiar?
Magnetic damage is amazingly effective against Corpus. But then, the Corpus are a faction that consists heavily of robotic and mechanized forces. And we all know what magnets do to computers.
Valkyr's Hyteria skill darkens your vision, forcing you into melee combat. Exactly what a real Valkyr would do.
All of Stalker's weapon are designed specially to kill warframes. His bow Dread and his scythe Hate, the two weapons you're likely to face in combat against him, focus on slash damage whose status effect is Bleeding.
Try not to think about the implications of finding Nano-Spores* "Fibrous technocyte tumor. Handle Infested tissue with caution." or Plastids* "A disgusting nanite-infested tissue mass." on Grineer or Corpus units.
Or the fact that Medium Team Heal Restores are made of the aforementioned Nano-Spores.
All of the medium and large Team Restores use them. It's possible that once removed from the "control" of the infestation, samples of the virus could be used to replicate material, but that has its own issue.
Survival missions are full of this:
During the event that first introduced Survival missions, there was no extraction. You fought until you died. How many brave Tenno did the Lotus abandon to their doom?
Corpus MOAs are bipedal robots, basically guns on legs, and the only sound they make is an odd warbling noise. But in Survival missions, they sometimes drop personal life support modules. They also do take toxin, gas, and viral damage (albeit less than other enemies). Which means they aren't robots.
Actually, just because they aren't machines in the normal sense of the word (like what most of us consider to be a robot), doesn't mean they aren't robots. In all likely hood, the parts that need to move the most (the legs) are organic in nature, like the Geckos from Metal Gear Solid, and would thus require atleast some oxygen to function normally.
Alternatively, they could just as easily be carrying spare tanks around for the troops like the proposed US Army M.U.L.E UGV. When one of the troops need more air, and his friendly neighborhood MOA can supply some.
As of Update 15, this is no longer the case in Dark Sector survival missions; instead, once the alarm is triggered, you're informed that the Infested are saturating the location with toxic spores. More fitting, yes, but not any more comforting.
The Valkyr is a Warframe that was a survivor of the Gradivus Dilemma, an event where Corpus exec Alad V disassembled several frozen Tenno to create his own special proxy, Zanuka, and the design of her Warframe reflects that. What appears to be some sort of mask is clearly the true form of her helmet, the inside plating quite visible in contrast to the flowing exterior, and a similar section along her spine shows the fine armor beneath the suit. In other places, what used to be the aforementioned organic exterior of the suit has been put back together with makeshift repairs, giving Valkyr's armor a very flayed and raw appearance. Furthermore, the rest of her suit is studded with pins, some of which seem to go through her arm, which could potentially be holding what's left of her exterior together. Combined with the obvious restraints that she now uses as weapons, and the constant anger and rage she barely keeps control of, and one can only imagine what horrors the Corpus did to her...
Though it's also a case of Fridge Logic if you see Warframes as classes of armors worn by Tenno rather than each as a unique character. Because it would mean the Tenno essentially replicated a terribly damaged frame including replicating restraints that go through the arms of the wearer.
The 'Valkyr's Bonds' syandana's description mentions that it was made in memory of the torture the original Valkyr went through. It could be that the Tenno, being big on honor, consider impaling themselves in the same way she was another way of remembering her pain. Alternatively, it could be that the recreated Valkyr armor's restraints only appear to be going through the arms. Or maybe what the Corpus did was somehow responsible for Valkyr's incredibly high armor strength, and the Tenno thus copied it perfectly.
Rescue 2.0 (introduced in Update 13.2.0) has Infested in prison cells. It's possible that the technocyte virus works so quickly that some random civilian, insubordinate Grineer, Corpus POW, Red Veil member, etc. was thrown in just 1 minute ago, and transformed into an Infested in as much time.
The event entry for the end of Arid Fear has Vor note that "reactive dissolution" hides the answers about Warframes. Considering that the Sentients turning the Orokin's own technology against them was a huge problem, it makes perfect sense that the Warframes would be specially engineered to prevent anyone meddling with them from learning the truth about them.
Which apparently doesn't work very well, considering Corpus dealing in Warframe parts, explicitly showing how they're obtained in the PS 4 trailer, and Zanuka boss/miniboss being made of said parts.
maybe because the Corpus are working from (admittedly hamfistedly-reversed-engineered) the orokin's own tech, and the Grineer mostly use much more crude tech that they developed themselves? or maybe Alad V's lack of problems with dissolution is plot-relivent at some po- oh god. what if the dissected tenno's warframes didn't self-destruct because they never DIED?
Which makes Zanuka/Harvester even more troubling.
One of Captain Vor's comments in the Prologue section of Update 14 implies that the Tenno do not just wear the suits around them, which give them unique powers, but are, in fact, beings of energy that rely on the suits for containment. While probably just the incoherent ramblings of an Orokin-obsessed madman, or excessively-metaphorical language, this could provide a logical explanation as to why the Warframes can manipulate the elements, create antimatter and gravitational shifts, conjure up nanobot swarms and ablative armor from nothing, and even why their "health" pickups are so similar to their energy pickups. The Warframes themselves are symbiotic conduits for their energy-based users, changed by their exposure to the Void, that let them channel that energy in specific ways.
Wow. That is... that makes absolute sense to me. It would explain why we never see the Tenno out of their Warframes, as well as the other things you have mentioned.
If I remember correctly, Vor's wording is. "The Tenno don't somehow control the divine light that gives them such powers. They ARE that light!"
The only question that brings up, though, is why the Tenno bleed when they take slash damage?
Could be internal fluids that keep the warframe (the suit, not tenno) running.