Trump's Democratic foes - and some Republican hawks - see dangerous signs of a president without much grasp of foreign policy who could be played by North Koreans.
Even Trump's own intelligence chief, Daniel Coats, testified last month that Kim isn't likely to give up his nuclear weapons.
But there are others - with deeper knowledge into North Korea than most - who say Trump's approach is the best game in town at the moment. They see his willingness to rip up the foreign policy rule book on a gut instinct might - just might - be an advantage.
They include Robert Carlin, a visiting scholar at Stanford University who was involved in US-North Korea talks from 1992 to 2000, and Siegfried Hecker, a leading nuclear scientist who visited North Korea's main Yongbyon nuclear complex in 2010.
At Stanford, Hecker, Carlin and researcher Elliot Serbin have been charting the degree of risk on the Korean Peninsula since 1992, using a range of indicators ranging from diplomacy to various aspects of North Korea's nuclear and missile programme.
On their colour-coded chart, bright green is the safest classification, bright red the riskiest.
When Barack Obama took office, the boxes were a mixture of pinks and light reds. By the time he left office, eight out of 11 boxes were bright red, with North Korea testing missiles and bombs.
By 2017 - with Trump's bombastically calling Kim "Little Rocket Man" - nine boxes were bright red.
"The risk of war was high," Hecker said.
Since then, though, the diplomacy box has shifted to green.
With North Korea suspending nuclear and missile tests, other boxes have returned to a more reassuring mid-red or pink.
"Rapid North/South rapprochement and the Singapore Summit in 2018 dramatically lowered tensions and the threat of war on the Korean Peninsula," the trio says in a report, "creating space and time to pursue diplomatic solutions."
Before leaving office, Obama told Trump that he had pushed his military to develop plans for a pre-emptive strike to destroy North Korea's missiles, Wit said, although those plans were never drawn up.
Wit said his conclusions were drawn from 150 interviews with Trump and Obama administration officials over the past year as part of research for a book.
Even Wit - who likes Trump's approach - says the President's iconoclastic style and resistance to criticism might also work against him.
Combine that with a lack of attention to details, and "that leaves him open to making big mistakes," Wit said in a meeting with a group of South Korean lawmakers and experts.
By now, Trump's negotiating strategy is becoming familiar, from NAFTA to the trade talks with China: Go in with all guns blazing, dial up the rhetoric and then finally let officials quietly seek a reasonable compromise.
The risk with North Korea, many experts say, is Trump's rush to make a deal at talks this week.
He could end up granting Kim the diplomatic recognition and economic relief he craves without forcing him to surrender his weapons, allowing North Korea to take its place as a de facto nuclear power, critics fear. He may also undermine US alliances with Japan and South Korea.
But Trump is defiantly sure he's on the right track.
"So funny to watch people who have failed for years, they got NOTHING, telling me how to negotiate with North Korea. But thanks anyway," Trump tweeted yesterday, before leaving for Hanoi.
South Korean President Moon Jae In, who has been the strongest advocate of engagement with the North, struck a similar note.
"Even after overcoming difficulties to get this far, some people are still displeased with improvements in inter-Korean and North Korea-US relations and are trying to drag them down," he said in a statement. "I urge all of them to discard such biased perspectives, and let's do our best to seize the opportunity approaching us."
North Korea's state-run Korean Central News Agency joined the pushback against Trump's critics.
Democrats in the United States, it said, are trying to "overtly and covertly disrupt the talks," using "all sorts of groundless stories and misinformation," KCNA wrote.
"Opposition just for the sake of opposition," the news agency added.
And the KCNA's take on the US intelligence community?
"It is absolutely as foolhardy as expecting to see a chicken turning into a phoenix to expect proper comment from the US intelligence agencies," KCNA wrote, "as they have it as their basic mission to claim white to be black, and lies to be truth, out of their scepticism toward others."
In Hanoi, the broad outline of a possible deal began to take shape, involving a declaration to end the 1950-1953 Korean War, the opening of liaison offices in each country's capital, the closure of the Yongbyon nuclear complex and some marginal sanctions relief.
But experts said it remains far from clear whether Trump and Kim will find enough common ground to get it over the line.
The outreach with North Korea is "clearly better off" with Trump's top-down summit-driven approach, said Joseph Yun, who served as the US special representative for North Korea from October 2016 to March 2018, spanning Obama and Trump.
"There is no talk of war, so everyone is supportive of engagement of North Korea, political, diplomatic, economic engagement," he said
"The question that remains: 'Has it made a big difference in nuclear weapons development?' So far the answer is no," he added. "So this is the question that this summit has to answer."
