Flanders cobbles are like tarmac compared to anything at Roubaix. I did the entire Flanders course last week, very fun. I went over to Roubaix just to check out the velodrome and try out one section (3 star, so not even close to the really bad ones) and it was like an order of magnitude worse than anything at Flanders. I think it is a misnomer to talk about the "cobbled classics". Paris Roubaix is an absolute beast (and I say that based on riding one section of cobbles, it was brutal) and is in a class by itself compared to Flanders. After having a full serving of Flanders and a taste of Roubaix, they are completely different races, especially with respect to the cobbles.

Cobbles at Flanders and Gent Wevelgem were bumpy (but they were mostly all uphill at lower speeds), but overall there are not a lot of cobbles in these races and besides being a bit bumpy and slower going/more effort they really weren't a big deal. They add some "character" to the race, but they don't seem like any kind of game changer and it didn't seem like there was any special skill to riding them, just need more power.