I think it sort of goes two ways. A lot of Labour (their shitty leader not included), and some remainer Tories, don't see the deal as 'soft enough', they'd prefer a second referendum, or a more permanent solution to keeping the UK in the customs union than the backstop. The Brexiteer Tories who want out out, and don't give a toss about Northern ireland or the economy or the GFA saw it as being too soft and hated the fact that it kept them tied to the EU with no time limits.
The problem with May's deal is that it was so in the middle that it didn't satisfy either leavers or remainers. Leaver's thought it kept them too tied to the EU with no guarantees of actually getting out, remainers thought it was just like the current deal without any of the benefits of actually being a member. It was a lose-lose for everyone, which anyone with half a brain could've seen coming, but ... UK parliament, bunch of feckwits.