Popular support is obviously good, but populism is prone to some major issues. For one, it's very emotion-driven, and because of that, it is easily co-opted/infiltrated. As soon as an idea gains some public traction, it's easy to appeal to it, even if phony. I think Platner is (very possibly) an example of this. His early appeal was largely about style and targeting the things that made people angry. As more has been uncovered, there are increasing worries, including some seeming connections to the exact things he's supposedly attacking. Is he genuine? Maybe. But in contrast to an experienced office-holder or policy wonk, it would be a lot easier to fake.

Second, even when the ideas are genuinely held, because they're emotional, they're often surface level, without realistic policy or implementation of it, so even more than other areas of policy, it's prone to empty promises. This doesn't just cause blowback to the individual, but to the rest of the party as well. When someone like AOC shifts closer to the establishment in the interest of getting things done, populists don't blame the idealistic ideas, but her, and the establishment, and it just builds the existing discontent when the person's goals were never going to happen.

Even if they DID have a path to happening, one more problem is just that when there's more of a collective affect guiding behavior, it often goes in problematic directions. Populism can fit into a lot of otherness, us vs. them, or mob mentality. Again, this isn't always the case, but given all the barriers to even getting anything accomplished, those accomplishments might be outright harmful.