Pros
It is good to have experience with wearables as well as mobile technology. Striiv has built multiple wearable products and has a CTO who has deep experience with firmware and hardware development. While it has been bad there for years there have occasionally been efforts to turn it around. The positive reviews on this site are from managers so keep that in mind when you read the reviews.
Cons
The work is very disorganized and learning is a low priority. Initiatives are started but rarely finished. The engineering team is actually criticized for attending meetups which means learning is essentially not valued. Overall it is a bad environment. There's been a high turnover rate which means that there are few people who have knowledge of what the app is supposed to do and what needs to be done to improve it. It is hard to do any work when nobody has the necessary knowledge about the app. And there is nothing written down which would help new hires get up to speed. There's a lot of resistance to making the necessary changes or apply modern technologies which means the code base is out of date by multiple years. By working at Striiv you will learn bad habits and won't gain any experience with modern technology. Your career will actually be damaged by working at Striiv.