From the Wrap-up of the article Effective Class Delegation:
implementation by delegation - has been named as the “worst” feature in Kotlin by the lead language designer, Andrey Breslav on several occasions (e.g. during the KotlinConf 2018 closing panel discussion). There are some cases where this kind of delegation can get complicated and produce some… Interesting behaviour.
What problematic cases exist? They are not mentioned in the article or the closing panel discussion.