I've noticed people on the Left (far-Left) ratcheting-up the anti-Warren language. They see Warren as a lot better than Hillary, but still too "corporate" and "neoliberal". E.g. Matt Taibbi said the other day during a "Useful Idiots" podcast that Warren was much more pro-business and capitalist compared to Bernie. His preferred candidate is Bernie. If Bernie doesn't get better, then the far-Left part of the progressive movement may be faced with a choice of voting for Warren or not voting at all. I would guess maybe 10% to 20% will choose the latter, based on ideological and purity considerations.
"If you can't do perfect, don't even try."
You have to remember that "far-Left" in the U.S. (without the authoritarian streak) equals "center-Right" in, e.g., Europe. Warren would be further right, still.
Generally, the consensus is that Warren seems to have good political instincts, but some horrible advisors - making her kind of flip-floppity and calculating, also with giant blind spots on foreign policy. Here are some reasons I have heard for why leftists prefer Sanders over Warren:
* Warren voted for Ben Carson to be the head of Housing and Urban Development, then flipped in the next round of voting after a massive backlash from supporters.
* Bernie wants to fully eliminate medical debt; Warren doesn't.
* Bernie wants to eliminate all student loan debt; Warren has a plan to eliminate some of it.
* Warren voted for Trump's bloated military budget; Bernie didn't.
* Warren has raised a lot of money from the military-industrial complex and engaged in pork barrel politics with Raytheon back in her home state (MA).
* Warren waffled on Sanders' 'Medicare for All' bill, which she co-signed, citing "lots of paths to get there" - i.e., Medicare for All/Extra/America/public option. Therefore, considerably less likely to pursue single-payer, but rather, "increased access".
* Warren has said she will not take big money in the primaries; but will do so in the General. But the dynamics of bought influence don't magically change in the General. What's worse, Warren wasn't even being truthful about the first point, having transferred over $10MM of the big money she raised in the 2018 Senate race to underwrite her 2020 bid.
* Warren endorsed Hillary over Bernie in 2016, even though Sanders is much closer to her ideologically. Again, this is telling of a much deeper level of calculation going on. It is possible she was vying for a VP spot (a great Machiavellian 3D chess move by Hillary, btw!).
* Warren embraces the language of bipartisanship, as opposed to class warfare. Made a 'spirited defense' of Blue Dog Joe Manchin in a TYT interview. Bernie, on the other hand, has gone on record saying he would use FDR-like tactics to fight for Medicare for All, including primarying other Democrats.
* Warren is a standard neoliberal on foreign policy: Implied Iran broke deal she voted to unilaterally violate terms of, will not condition aid to Israel, will not ease sanctions on Venezuela. etc. etc.