I'm going to try and give a more concrete answer to this rather than just talk about principles to illustrate why I'm sceptical.
Taking Bernie Sanders as an example. Although I can agree with much of what he says, his political positions bear little or no relationship to the fact that the United States has a two-party system. Whilst focusing on policy is good in terms of substance, he's relying on convincing people that the Republican Party will not affect his or his party's ability to implement a radical agenda, effectively assuming they won't oppose his policies and will not simply try to reverse them as soon as he is out of office when he's campaigning.
The Affordable Care Act/Obamacare wasn't Socialist, and look at the level of resistance and hysteria that generated and the threats to repeal it as soon as a Republicans got back in control of Congress and the White House. It is very hard to imagine the same thing not happening if Sanders became President. All this assumes that Sanders would have complete support within his own party- which is unlikely given the dominance of the Clinton-Obama "New Democrats" since the 1990s.
The flip side of this, is that Sanders couldn't by pass the Republicans because of the separation of powers and the way the U.S. Constitution works. As President, he couldn't just write an executive order and say "hey look, you all get free health care now". It get challenged in the Supreme Court (much like many of FDR's new deal programs were and his attempt to "pack" the supreme court). You'd have to control both houses of Congress, the Presidency and appoint a majority of supreme court justices to pursue a radical socialism program in the United States.
And this is all assuming that there isn't a potential coup or assassination attempt by the military or the intelligence services who have lots of friends amongst the business community given all the private contracts they have and lots of practice of overthrowing left-leaning democratic governments overseas when it suited them. (Again, there were allegations of such a conspiracy against FDR or maybe possible CIA involvement against Iran (1953) or Chile (1973). And of course, there is the less obvious options of trying to impeach him through Congress, passive resistance within the Federal government by not fully or urgently implementing his policies, or using the 25th amendment to replace him with a vice president (who may be more "sympathetic" to the concerns of the business community).
And this is still assuming the Democratic nomination is a fair fight, and that the presidential election is not going to be driven by wealthy donors using their money to defend their privileges and a hysterical mass media shouting "socialist" and making unfair comparisons with the Soviet Union, China, etc because they have only a tenuous relationship with truth even when it suits them.
Now, take a close look at Sanders; he's been a a mayor for 8 years (1981-1989), a member of the House of Representatives for 16 years (1991-2007), in the Senate for 12 years (2007-2019). You've got two options with Sanders:
Option 1 is he doesn't understand how democracy works in America despite serving over two decades in public office, or is so utterly blinded by his good intentions and belief in the american people he cannot see the limitations of working in a constitutional system of government with a two-party system;
Option 2 is he doesn't really care about Socialism and there is a hidden agenda at work and the socialist rhetoric is just a smoke screen for what he's actually doing.
Where ever you try to implement Democratic Socialism, you get a variation of this problem. If you are a Democrat, whilst claiming to serve the people and derive legitimacy from realising their interests, it also means you may not get voted in, you will have to face resistance to implementing your policies or they will be overturned when you leave office. If you're lucky, you might get a Scandinavian situation where Social Democratic Parties hold power for so long it's effectively a one-party state, but if your a socialist and you want to kill capitalism and keep it dead, it's nearly impossible to support Democracy at the same time.
