Using @Entity is not really dangerous, anyhow, whatsoever. I will try to address your points one by one:

1) "as you don't know how it can be used.." - but wouldn't same "problem" be in place in case of DTOs? even though I'm not sure what are exact points of misusing, what if you also misuse DTOs? I think, you can misuse any component of any layer, but that does not necessarily make returning Entity instances, dangerous.. that's why you should avoid designing your software in a cross-cutting fashion;

2) "Being reactive on its nature - you can accidentally update some data inside Entity instance and this change will be persistent during JPA "save" action."

- Well.. 2.1) again, you can do many things accidentally, and the problem there is such accidents (that involve "accidental update" followed by "accidental save"), as such..; 2.2) updating accidentally DTO can also cause the problem, if you aren't careful enough.

I really think, that these are different scopes of responsibility.. and DTO, as its semantics suggest, should be responsible for Data Transfer.. considering clean architecture design, we should also stick with Single Responsibility Principle (SRP).. that is, the responsibility of DTO should never be a safety, per se.. but the Data Transfer, only. Otherwise, even if your code-base doesn't grow, your conceptual model still is going to be a bit ambiguous and overloaded.. as you care about transfer and security, at the same time.