The issue with it isn't so much that it isn't safe but that is nowhere near as optimized as the 787 and A350 not to mind narrowbodies like the 737 and A320 that the C919 competes with. Optimized in the sense of "least cost" operations, particularly fuel consumption. Also optimized in the sense that Boeing has been making 737s for 55 years now so they are really good at making them and can pass the savings on to their customers.

The C919 has the first prototype built in 2015, flew for the first time in 2017, was approved in 2022 and entered commercial service in 2023. There is no sense in which development was rushed.

https://en.wikipedia.org/wiki/Comac_C919

You can’t fly it to prove safety to FAA or EASA. It needs a ton of build and test data to show it’s been designed and built correctly. Also those agencies don’t allow use of stolen data so you have to replicate all the material allowable testing to get credit for it. Airbus had to do that when they started up. They stole all of the design data from the us and then had to slowly reconstruct it to launder it into an acceptable form. The Chinese are doing the same thing and will need to complete the process to get FAA and EASA approval for their models. The process is sort of like doing your homework when you have the answer key, it’s simpler than without one but still quite time consuming.