Keep in mind that the upper and lower halves of Summoner's Rift are essentially mirrors of one another. So while a Rek'Sai on the blue side has easier ganks from bottom side tribush into bottom lane, the Rek'Sai on purple side has easier ganks from the tribush topside into top lane. The only reason the ganks appear to be better for a blue side Rek'Sai is because a gank on the bottom lane is a potential double kill, or at least two kills for your team if they go to different people, and there are two assists going to the other two teammates who don't get the kill on whatever champion died.
In other words, Rek'Sai ganks bot, gets one kill, and Vayne gets one kill. Thresh gets two assists, Vayne gets one assist and one kill, Rek'Sai gains one assist and one kill. Overall, if both enemy laners are killed, a Rek'Sai who ganks bottom lane will make more team gold than a Rek'Sai who ganks top lane as that will only result in a single kill and a single assist.
This is where the strategy and positioning for Rek'Sai come into play. Yes, a blue side Rek Sai might have a better entry into a pushed lane to gank, but his ganks are also weaker top lane, where you can instead focus on getting your top Riven or Nasus fed. And even if your entry into the bottom lane from purple side isn't as good, by tunneling across the wall into the bush on the purple side of the bottom river, and using her high movement speed when burrowed to get under enemies, you can still knock them up and try to help your laners get the kills. It just won't be as easy because you have to do a bit more moving.
That's just part of the strategy though. Just like how playing on the blue side of the map gives you a very slight advantage when it comes to sight on your screen since you aren't having to contend with the UI being partially in the way of your view like purple. Yes, this is something that is noticeable, but it isn't the reason that a game is won or lost as it isn't anything that proper positioning and engaging can't overcome.
Whatever small benefit is going to either side of the map, it's still up to the players to use what they have and it's just as easy to misuse an advantage as it can be to make use of your own skills to overcome one.