This exploit has been around for ages. I've heard that people have gotten soft-banned in the past for it, but it's infrequent. I've been using it for months and haven't gotten penalized for it. The main reason I started using it was to better control when I was getting coins from gyms so that I wasn't wasting effort defending. Getting raid passes and field research breakthroughs a day early is a nice bonus. I don't see it as too game-breaking, since you can't get 'extra' bonuses out of it - you just access things one day early.
The biggest boost using this exploit has given me (and others) is that it allowed you to easily collect 11 free shots at Articuno and Zapdos. Save Friday's pass, do the 5 for Saturday, then jump ahead to a time zone where it's Sunday and collect 5 more. I was expecting that if a crackdown was going to come, it would have been after Articuno Day when people were getting double the intended free shots at it. Since they didn't, I don't think they really care that much - still, use at your own risk.
