First and foremost, it is crucial to stress the importance of doing your own research. Even something as simple as performing a few Google searches will probably prove to be more than enough.
No matter which cryptocurrency exchange you are thinking about using, prudence is the operative word and performing your own Due Dilligence is essential.
That being stated, here are a few Bitflyer case studies from Reddit alone:
https://www.reddit.com/r/BitcoinMarkets/comments/97cg7y/bitflyer_scam_bitcoincryptocurrency_exchange/ (a case study published by the owner of Bitflyer.Sucks, where he shares his personal experience with Bitflyer, all of the other case studies posted on this page have been published by other individuals)
https://www.reddit.com/r/btc/comments/97b9y6/bitflyer_withholding_funds_for_8_weeks_now/ (a case study which involves a very significant amount of money that is being fraudulently withheld by Bitflyer)
https://www.reddit.com/r/BitcoinMarkets/comments/9cr48g/bitflyer_class_action_lawsuit/ (the possible beginning of a class action lawsuit against Bitflyer)
https://www.reddit.com/r/BitcoinMarkets/comments/672oao/bitflyer_experiences/ (Bitflyer opinions)
https://www.reddit.com/r/Bitcoin/comments/8u7xyw/something_wrong_with_bitflyer_japan/ (individual Bitflyer concern)
https://www.reddit.com/r/btc/comments/8u74cm/something_wrong_with_bitflyer_japan/ (Bitflyer Japan concern)
https://www.reddit.com/r/bitflyer/comments/7qm1mm/bitflyer_experience_reviews/ (Bitflyer opinions)