As populism is at the bane of any politician (you gotta say/find out what your people want/want to hear) but i gather you mean populism as a modern political entity.
"Populism" is often mainly criticizing the status quo but not offering any credible detailed plans / policies to make good on their "policies and "rhetoric"
Just look at how quick Farrage and everyone who campaigned "Pro-Brexit" backed away from the project when it got a "yes" result. They knew they did not have an operational agenda as to "how" the brexit was to be achieved. So now people that don't want a Brexit, and never planned for a Brexit, are forced to handle and achieve Brexit. Hence the UK being in a constant state of political crisis for the last 2 years.
As another example: Dutch parliament member Geert Wilders his PVV has a party program of 1 A4, with mostly vague promises/points without any reasoning, planning or calculations. As an example he wants to get the Netherlands out of the EU and out of the EURO. But he has not offered any plans how this would be accomplished , has no calculations done about the costs this project would cost, the impact it would have on Dutch businesses and citizens or how much extra costs will be made in money exchanges when this would be done. These things are pretty important for a small country relying heavily on trade within the EU. I don't trust a politician (wilders) who says "everything will be alright, just trust me", show me the plans and prognosis instead to make your point.
As a third example Trump: He never told/showed his plans during his campaign or debates. (he liked to keep his opponents in the dark.....) He mainly made big vague promises, criticized obama, but never told or laid out how to achieve his goals or a bigger picture. (oh yeah mexico would pay for the wall). He passed a handful of promises and is trying to pass a few more to appease his voters, but is leaving many other issues completely untouched (infrastructure for one). From his acting over the last 1,5 years you can also see there is no bigger picture, grand strategy or "4d-chess" planning. Its all done "on the fly" and "from the gut". If there was actually good planning and strategy Donny would have a much easier time getting things trough the republican dominated senate and congress for example.
edit:
This is also my main problem with "populism" in general. I just like to know how they will fix things and achieve their plans, but they quite often can't reason further then a few quick oneliners or deflections.
@Boukert has some strong opinions about this.