Congressional objections to IPAB seem to focus on the potential power of the un-elected IPAB board. If IPAB is implemented, Congress may not control the purse strings when it comes to Medicare. Stakeholders such as the powerful American Medical Association aren't too thrilled with IPAB's potential to cut provider payments.
Meanwhile America's Health Insurance Plans objects to the exemption of hospital costs from IPAB consideration until 2019. On the other hand, the American Academy of Actuaries has crunched some numbers and thinks IPAB needs more authority to help move Medicare toward a more sustainable financial model.
What Congress might really do
Congress is focused on repeal when tweaking is probably what's needed. Both the Senate and the House have bills in place to repeal IPAB. More than half of the House members, including some Democrats, have signed on as co-sponsors of HR 452. The House bill may gain some traction, but that won't make any difference unless the Senate can get something bipartisan going.
S 668 was introduced in March and sits in the Senate Finance Committee. The bill has 32 Republican co-sponsors and zero co-sponsors from the other side of the aisle, which means it will probably go nowhere.