Authorship for paper with questionable statistical analysis For my master thesis in clinical medicine I have collected and analyzed data for a retrospective cohort study. Now one year later, a research assistant of the group is about to submit a manuscript based on my master thesis and I have been offered shared first co-authorship with said research assistant. Upon internal review I have noticed the following issues:

Definitions of clinical outcomes of the study have been changed such that they are now not well defined and not even of clinical interest
The statistical analysis I had performed for my thesis was completely ignored and omitted in the manuscript. For the manuscript, the research assistant employed statistical models I deem inappropriate as confirmed by fellow researchers external to the group. In addition, reporting of methods and results is poorly written with critical information missing.

I have provided feedback concerning the statistical methods but it was ignored. My advisor refused my request to see the new statistical analysis (in particular, I asked for the statistical software files edited by the research assistant), telling me that even though I have shared co-first authorship for the paper, I am not entitled to any insight in the statistical analysis, since the research assistant is in charge of it. I’m having a hard time accepting publication of a study of which I’m co-first author but haven’t seen the analysis behind it, especially since I highly suspect it is inappropriate. My advisor threatened to remove me from the paper completely if I don’t agree blindly.
Shall I renounce and let it be published without my name? Shall I agree and accept to have a paper published with my name and the knowledge that the quality might be really bad? Shall I contact the editors of the journal they want to submit it to?
Edit: Thank you all so much for your thoughtful answers and comments! I appreciate your help!
If you are to co-author a paper, you need to agree with its methodology and conclusions. You don't.
My advice, in general, is to offer to work with others to improve the paper to meet everyone's standards. You indicate that this might not be possible, which would be a fault of the PI. If you can't convince them to work with you, then I don't see a real way forward without saying that you won't be part of it in any way.
They can then go forward if they like, citing your past work properly. If they don't do that, it would be time to contact editors.