It is very difficult, but it depends on the greatness of the idea and rigor of the research. One of my colleagues is an early career psychologist and published recently in Nature testing an intervention in a national experiment for youth at a critical transition period.
It is certainly quite tough for junior researchers to publish an article in Lancet or Nature, but, in my opinion, the actual difficulty may depend on a wide range of factors (for example: your specific subject/area of interest, to what extent your research findings can be considered "ground-breaking", whether in your team there is someone with an established reputation who has already published in these journals, etc.).
With regard to Nature, you could benefit from taking a look at two conversations in Quora and Academia on a similar topic:
Dear Jacky Bhagat from my personal experience, it is VERY tough if not impossible to publish in journals like Nature or Lancet. This is particularly true for early career researcher who are working alone. In order to publish in these journals you need to have exciting results of significant novelty AND broad interest. Otherwise you manuscript will be desk-rejected right away. Even if it's a poor consolation, I can tell you that during our 40+ years of research in inorganic chemistry we never published an article in Nature or Science. The reason is that our work is too specialized.