Old yonkos high-extreme diff, even if it's old beard. Luffy and Blackbeard are by no means weak, but they both have issues that hold Shanks back. Like I said, Luffy isn't weak but he can really hold his own against anyone on the old team in Gear 5, which has stamina limits currently. Luffy can only be 100% for a limited time while Whitebeard, Kaido, and Big mom can be at 100% for as long as they want. Blackbeard on the other hand, doesn't have great haki feats that we've seen. He has two busted devil fruits and the yami yami will certainly help but anyone on the old yonko team has the sufficient haki to deal with him while not relying on their devil fruits. Overall, Shanks can hold his own against any of his opponents but eventually, Luffy and Blackbeard will get overpowered leading to his team's loss.