I believe that Rayleigh is, individually, around as strong as Whitebeard. I don't really mind if barely weaker or really equal, but they should be around equals, mostly because everyone that's that strong is kind of like Akainu and Aokiji: if two individuals have to duke it out for days until one wins, none of the two is truly superior to the other. Not even the winner.
People currently praise Beckman for being the right hand man of Shanks, a man who with a small crew holds the same power and strength as the other yonkos, which implies that his crew needs to be very damn strong and, at least one or two of them, must be close to his own level or they'd just be attacked and overpowered by someone like Kaido and his army.
Now, think about the Roger pirates. They were just as few, if not fewer, than the Shanks pirates. Of course when people discuss the God Valley incident, all the focus is on the man who became the King of the Pirates and the man who became "the Legendary Marine", who teamed up together to take down an alliance of the men who later on became the Yonkos (+ Xebec + Shiki + probably other crewmates of their own level that may have died on that day). It is impossible for Roger and Garp to really hold of the merit for succeeding and I honestly do not believe that the two of them together were strong enough to defeat all those powerhouses, precisely because all those monsters are around the same level, and Xebec may have been even stronger. It's imperative for Roger and Garp to have been helped, and it probably wasn't only Rayleigh, but Rayleigh is definitely the first man that jumps to mind that was there that could have taken on one of the future Yonkos in that battle. Let's say that he didn't defeat a future Yonko that day (and there's no reason to believe that he didn't): he should have at least stalled one of them long enough for his captain to defeat Xebec. A man who can do that needs to be very damn strong.
There are still many mysteries surrounding Rayleigh in the manga: we can tell that he was a nobody before he met Roger (since Roger recruited him in a very similar way to how Luffy recruited Zoro, who was also a nobody back at the start of the series), yet for a man who's now only known as a "second in command", his epithet is "the Dark King". There must be a reason behind such an epithet. All epithets throughout the series are put for a reason, and it doesn't really make sense for a man to be called "King" when he wasn't even a captain, so I'm sure that at his prime he made or showed something to the world that made him earn such a name. It's even weirder if we take into account that his captain was the "King" of the Pirates that he would still be called "King" himself, and I'm sure that there will be an explanation for that in the future, and it will revolve around just how damn powerful he was as well.
I could also go on about how in his prime he was a swordsman who dressed just like Mihawk dresses now and I keep betting that he's Mihawk's master, which indirectly hypes up both of their strengths, and it would explain how is it that Mihawk and Shanks became rivals in the first place.
So, yes, Rayleigh is that much of a threat, but as it happens with Mihawk, he does not have a crew, and that's a huge point, because Yonkous are not only overpowered pirates; they're overpowered pirates with huge crews to support them. Make Whitebeard walk into Marineford all alone and he wouldn't even make it to the plaza despite his power, but make Whitebeard, all of his commanders and allies walk into Marineford and he would have succesfully rescued Ace if Ace hadn't turned around in the last moment.
Throw Rayleigh, Mihawk or Shanks alone there and the same happens, they don't make it to the plaza, because they're outnumbered not only by weaklings but also by pretty strong and annoying characters like Jozu and by other monsters like Akainu, Garp and Aokiji. Numbers matter a lot.
There were many characters that held back during the Marineford war, particularly the Shichibukai and some of the pirates, so it's not like they couldn't have also stopped Rayleigh (who is way past his prime even if he remains a monster) if they tried harder. Aokiji and Kizaru in particular were still very fresh and they're both monsters. However, there would have been a lot more of casualties from the marines; perhaps even an admiral would have died or become maimed.
However Rayleigh would have never been able to stop the war like Shanks did, precisely because Shanks brought his crew, together with Beckman and other monsters. That's the main difference between these characters.
