No, people who see things like this:
Realize he is unelectable because after 6 months of Republicans hammering the airways with it no one will vote for him.
Well, that goes with the "unserious" point I mentioned. The scope of political discussion is so skewed in this country that any tax increases are considered "unserious ideas", regardless of whatever benefits are actually offered by them (or even whether the taxes are calculated correctly!). It seems like it's ok to advocate for public, progressive ideas (which, by all rational observation of the evidence available worldwide, will require some degree of tax increases, and not just on the rich), but we don't want to do it too loudly, or else the Republicans will hurt us!
Instead of finding a way to actually sell how those tax increases would benefit American society (and in a lot of ways, save us money in the long term), it seems we'd rather just avoid the issue and kick the can down the road some more. So more tax increases only on $250,000+ incomes, even though that likely won't solve the actual problems we claim to care about.
Because after all, the Republicans are coming! Which is certainly a fair argument I suppose, but we've been electing "serious" Democratic candidates for the past 30 years, and the terms of debate are still framed and dominated by right-wing terminology, and certain issues (such as income inequality) have continued to worsen, regardless of whether Democrats or Republicans are in office. In fact, some Democrats have actually consciously worsened those issues!
And note, I said "certain issues", not "every issue", so I'm not making a "Democrats and Republicans are exactly the same in every possible way!" argument.
Not sure what else to say. Some people like "truth-tellers" when they have relatively little influence, but get worried when they start to have more influence. I guess it's partially a fear-based argument ("yeah he's cool and all, but we can't risk a Republican, so he should quiet down for now"). Which is fine, but it's amusing to see when some supposed progressives then start using right wing talking points to criticize him. It often seems like the "he's unelectable" argument is the only thing to grasp onto, so when he starts showing he is electable, we then have to reach into other arguments to criticize him. And since you can't really criticize him from the left, that leads to the centrist and/or right wing talking points coming out. (Well, the hardcore left can certainly criticize him, but the average Clinton supporter isn't really hardcore left)
I'll clarify also that if someone just genuinely believes in Clinton's ideas more than Sanders' ideas, that's fine, and kind of a separate topic. I'm more speaking to the "I actually like Sanders' ideas more, but you know they're unrealistic, and he isn't electable, so Clinton is the way to go" segment of the population.