The reasonable criticism of Bernie's proposals is not that they're politically impractical, but that they are practically impractical -- which is to say, in general, they have not been spelled out enough to understand how they would be implemented, and in the cases where they were, they made unreasonable assumptions like very high GDP growth to reach reasonable funding levels.
This is actually a very notable critique -- as people have mentioned, Bernie has a strong record of getting amendments passed in the Senate. This requires a skilled legislative pen. Bernie should be capable of writing bills that could be passed as written. If he's choosing to push forward bills that can't be passed as written, it's appropriate to question whether those goals are practical or reasonable. After all, if he could write bills that just work, presumably he would be doing so.
Also the thing where he called open borders a Koch brothers plot was kind of notable to me, just saying