Larry Summers is a Democrat. He typically disagrees with Republican economists about tax policy. That is expected because Democrats and Republicans do not share the same value system. Even though he often disagrees with respectable Republican economists he understands why they disagree on tax policy and he does not question their honesty. Summers has no respect for Trump's plan. He explains why the Trump administration's tax plan is a terrible plan that is based upon dishonest assumptions by his Treasury Secretary and his top economic advisors. He describes the likely economic consequences of the plan, which will be bad for our economy, and bad for the majority of Americans. He concludes by arguing that the US will lose its position of economic leadership during the coming meeting of central bankers and international economists. They don't try to sell terrible economic plans to a poorly informed electorate by telling obvious lies. The fallacies in the Trump plan and the lies being told to sell the plan will be obvious to everyone at the meeting.
The Economic Policy Institute is a liberal leaning organization that does not share the value system of the Republican Party. However, it has carefully considered the assumptions that Republicans use to justify their tax policies. Most of the assumptions do not stand up well to a careful examination. In particular, the assumption that corporate tax cuts are required to stimulate corporate investment and job creation fall apart upon analysis.