The great majority of critiques of the Iran nuclear agreement are flatly dishonest and intentionally misleading. They'd be laughable if the stakes weren't so high. Others are well-intentioned, but tend to be either misguided, uninformed, or over-inflated. Or they skirt the issue of what a realistic alternative looks like. If you want to understand this issue, read Daniel Larison at the American Conservative. Go there and keep scrolling. He will swat down every criticism that you've heard.
Here's an excellent article by Kingston Reif at the War On the Rocks website.