1. The fact that UMN Biostat may not be as strong as the current ranking shows matters if the ranking is one of the most important part for your consideration.
2. Dr. Pan is advising several students in Biostat department and also a few (>=3, I'm not sure) students from Stat & ECE now (usually the students from Stat/ECE have more solid background when doing ML/DL, probably the reason why he prefer them?). I doubt if he has enough funding to support more people working on his ML project. Given he is probably the only one who is seriously doing ML stuff in the department, if you could not get along with his style, your situation might be a little bit awkward. I'm not familiar with him personally, but I heard he is more to the pushy side. Do not put all your eggs in one basket. I guess you could find more professors from BU stat/math interested in ML.
3. I do not think BU is less reputable than UMN, at all. Considering alumni network, I would say that BU is a little bit more recognized during job hunting compared with UMN. However, going to industry, once you've gone through the initial HR screening, the brand/ranking of your university would be no longer a big deal.
4. Enjoying yourself in your daily life is one of the most important part to live a happy, healthy and successful PhD life without depression (which affect 60% of the phd students here in the U.S.). You do not know what it means when you are saying 'suffer' with ease now.
If what you really want is a Stat PhD, starting a biostat PhD may not be a wise choice. UW is definitely a *big* name in Biostat area. If you don't mind to fund yourself, I would say it could be a great stepping stone to a better Stat/Biostat PhD program. If you are still hesitating on going to academia or industry, it also could be a period of time for you to figure it out with plenty of room to plan your next move. None of the choice is 100% perfect and can ensure 100% success in your future. You'll gain something and at the same time lose something, but everything'll be worth it in the end. We cannot make the decision for you here, just follow your heart.