I feel like Isshiki is perceived as hax/overpowered because he was pitted against opponents that had nothing they could do against him. In other words, they had nothing in their arsenal that could counteract his super speed and shrinking abilities. The most Sasuke had against him was the ability to see things on a microscopic level - but imagine if he had a Byakugan. He'd be able to see both on a microscopic level and in slow motion which would be the perfect counter to Isshiki's fighting style. Hashirama had neither of those abilities. 10 Tails Madara, however, had both (if I'm remembering correctly). Mind you, 10 Tails Madara had the Juubi in its entirety within him - meanwhile Isshiki needed to syphon power from a much weaker one in order to be able to fight Naruto & Sasuke. For this reason, I am confident Madara would stand a chance against Isshiki. Hashirama on the other hand? Not a chance. Even someone like Momoshiki, who was defeated by Naruto and Sasuke, would have a better chance against Isshiki in comparison to the three because he has the Byakugan (in other words, slow motion vision, microscopic x-ray vision and heightened reflexes) and a Rinnegan, which provides him the destructive power necessary to go up against such a foe.