The Land Raider in RoB had 95 structural integrity and was then nerfed to 42 after the errata. In the Only War beta, a Basilisk has 45, a Leman Russ has 70 and a Baneblade has a whopping 120!

Is it right for Only War vehicles to have such high S.I. when a Land Raider is considered to be one of the most powerful tanks in the 40K canon?

I don't have RoB with me, but I would contrast it with the amount of armour it has. The Land Raider just has more of it, which is effectively HP for the thing compared to the Leman Russ.

And yes, a Baneblade should have more

 That doesn't make much sense, the Predator has SI: 45, more than the Landraider….oops?

Well that settles it in my mind. The nerf must have been an oops if we compare ALL of the vehicles against each other.

 I don't think there's that much point doing cross-comparisons between Rites of Battle and OW, considering they are technically different systems, in this case the most important point being the relative difference in PC strength and standard weaponry. The other difference to consider is that vehicles in DW were an "add-on" since it was from a supplement, whereas OW seems designed from the ground-up to make Vehicle-based campaigns a solid option. I'm sure that was a factor when they went into a redesign of vehicles. 

 Well if you compare some of the chaos vehicles they have almost the same stats.

The SI numbers on vehicles could use a sound nerf. Right now, two Leman Russ tanks shooting at each other can keep doing so until either crew gets bored and calls it a day.