My take is that project-based learning works well when you already have the subskills and need to learn to integrate them, and badly if you need to acquire the subskills first. Traditional educators were well aware of this: an old-style university degree has you take individual modules and exams first, and then do a big dissertation at the very end.
A more scientific version of the claims here can be found e.g. in the paper "Why Minimal Guidance During Instruction Does Not Work: An Analysis of the Failure of Constructivist, Discovery, Problem-Based, Experiential, and Inquiry-Based Teaching" available at [1] (paywalled, other sources may be available) and a brief summary of the state of research in the field is available at [2] for free, with links to various relevant papers.
Note that all of these show that discovery learning (which more or less overlaps with project-based learning unless the project is very tightly guided) is ineffective for learning new things, but this is often rounded off to "discovery learning is ineffective". As far as I know, the research does not show that projects are necessarily bad to practice and deepen knowledge that you've already built up.
A more scientific version of the claims here can be found e.g. in the paper "Why Minimal Guidance During Instruction Does Not Work: An Analysis of the Failure of Constructivist, Discovery, Problem-Based, Experiential, and Inquiry-Based Teaching" available at [1] (paywalled, other sources may be available) and a brief summary of the state of research in the field is available at [2] for free, with links to various relevant papers.
Note that all of these show that discovery learning (which more or less overlaps with project-based learning unless the project is very tightly guided) is ineffective for learning new things, but this is often rounded off to "discovery learning is ineffective". As far as I know, the research does not show that projects are necessarily bad to practice and deepen knowledge that you've already built up.
[1] https://www.tandfonline.com/doi/abs/10.1207/s15326985ep4102_... [2] https://debunker.club/2015/06/05/discovery-learning-is-not-e...