The two men are yonkou, so their crew are two of the best crew in the world. But let's analize the main difference between them: Shanks has about 10 men in his crew, while Whiteberad has 1600 men. Even if we consider only WB's captains, he has a numerical advantage and since it's said that every commender of a WB's division has the strength of 100 men... well for Shanks it's hard to be compared. It is implied maybe that even Shanks friends are very powerful. But even in this case, well, are the others WB pirates really as strong as flies? 1600 vs 11 is not possible. All this is to try to convince you that Shanks crew is very very very weaker than Newgate's one. And so my next question is: if Shanks is an emperor he is much stronger than Newgate's allies who are not emperors, right? And the two others yonkou? Are they at the level of Shanks or the level of Whitebeard? There's no shame in Shanks being the weakest yonkou in my opinion.
Feel free to try to make me understand that I'm wrong.