How do you find statistics for PhD completion rates? By department or by advisor. That seems important for choosing between offers.
I don’t believe that data is uniformly published across institutions
the only publicly available information that I have seen is published by Duke.
Some other schools open the curtain during the application process, but it is a critical thing to ask.
You can ask about this. Ask the department secretary and/or the professor who you are chatting with about joining their lab. They should know that. You can also ask about average time to completion as well.