Yes, he is, for a number of reasons. His ideals are noble, and his plans fairly sensible, but as any rational person has come to realize, that's not what wins national elections, or makes a good president.

1) It's easy to take the positions he does when you're a Senator from a small, homogeneous, proudly independent state, who's been elected and re-elected with 65% & 71% of the vote, respectively. There's an argument to be made that Vermont is one of the least-representative-of-the-nation-as-a-whole states in the entire union - which is to say, he's never had to run a campaign that appeals to a broad swath of people. Look at almost every single president of the last 50 or 60 years - they come from either large/important states (Bush & LBJ f/ Texas, Obama f/ Illinois, Reagan/Nixon f/ California, Kennedy f/ Massachusetts, Roosevelt f/ New York) or held important offices which boosted them to the presidency (Eisenhower - Supreme Allied Commander, Truman - Vice-President, Bush 41 - Vice President/CIA director). Clinton is the exception, but he was a political genius. The point is that you win national elections either by default (vice-presidents who ascended to the presidency) or by having run, and won, in difficult, microcasm-of-the-nation-as-a-whole states. There's no precedent for someone as radical and untested as Sanders winning any major election.

2) Who's going to fund his campaign?

3) National campaigns are won on charisma, force of personality, and national sentiment as much as any of the issues. He could have a - to quote The Dude - "Swiss fuckin' Watch" of a plan to run the government and solve all of our problems but that won't mean a thing if he can't rally the base, draw in moderates, and convince the youth and minorities to come out in force for him the way they did for Obama.

4) Ultimately, pragmatism makes good presidents, not idealism. Obama might be the most "socialist" president in history, but even he has had to kowtow to the realities of the world, and the realities of the legislative process. He had trouble passing important legislation when the Democrats held both the House and Senate - which wouldn't bode well for Sanders even if he were so lucky as to have a favorable Congress, but bodes absolutely catastrophic if he doesn't.