I think you guys are too quick to judge whether Genos is wrong about Mad Cyborg or not. There are too many variables.
First of all, what Genos says doesn't necessarily mean that "It would be a tough challenge." It could easily be that Mad Cyborg is crazily strong by normal standards and Saitama would notice it.
Besides, Genos notes that Mad Cyborg "is particularly vicious" even according to Hero Association records. This can be interpreted in many ways. One could say he is truly someone with extraordinary strength. On the other hand, Vaccine Man was the strongest monster HA faced by the beginning of the series. But this statement could also be false due to insufficient data (HA was newly built) or concealed information. We know that "EC vs Blast" was hidden from the public and then there is "God". My point is that it can play out either way.
And the most important part is about whether Genos' perspective is distorted or not. For instance, most people think that Genos overestimates Mad Cyborg due to childhood trauma. That's fair. But Genos also has seen his own share of destruction and death. He constantly deals with stress and he also seems someone capable enough to judge such things from an objective standpoint. Even if we assumed that his trauma overrides these, he is also obsessed with Saitama. It is unlikely he would place Mad Cyborg so highly even with trauma.
All of this might very well be Genos' ego as well. At the very beginning, we see that Genos talks about how he thought "nobody besides him" could match Mad Cyborg. So if Genos thinks his BoS-self is as strong as Mad Cyborg, doesn't that mean that Mad Cyborg is actually weak? Or maybe he just placed himself on a high pedestal and saw just how wide the world is after his ego got shattered and now realizes the cyborg's true strength.
Genos' perceived strength of Saitama is another problem as well. We could actually ignore this one thanks to my first point but I want to expand on it. While Genos is obsessed with Saitama, he always gives a fair assessment of his strength based on what he has seen. He gets surprised by his "Death Punch" and later on, he seems surprised that Saitama can stop the meteor. During Alien Conquerers arc, he gets taken back when he learns that Saitama has been launched to the moon. All of these feats one-up the previous ones. So how strong Saitama is according to Genos? The greatest feat he knows of is the destruction of Jupiter. Tier 5/4 Mad Cyborg might seem a stretch but we saw how Awakened Garou ended up.
If we dismiss the Jupiter feat, we get more reasonable levels. Genos places Saitama at the multi-continental level. This might also seem out there for most but it is actually very believable. It seems to be the benchmark level where Saitama holds the faintest traces of memory monsters and MA arc Genos could already produce that level of firepower. Besides, if Mad Cyborg is one of the bosses of the Organization, it would make sense that he is at Orochi level or higher. The Organization has the most built-up in the series and has already analyzed MA. Metal Knight also seems to fear it more than MA and Drive Knight has a piece from Psykorochi.
Don't get me started about whether Genos' memories are reliable or not. In Season 2 OVA 3 we see how easily his memories can be altered. In "A Festival with Saitama", he says that his memories might be fake. In the most recent chapters, we see that his core also serves as memory space. Besides this, there are many ominous points about Genos across the series, with him being implied to be a monster on at least three different occasions. We even have the "Genos is the Mad Cyborg" theory.
So is Mad Cyborg really that strong? Will he get one-shotted? Is he Genos? Is he alive? Does he exist?
I feel like ONE is playing 5-D chess with us.
