Interface inheritance is OK (as long as you pay attention to these issues) but implementation inheritance is a huge mess because it's effectively antithetical to modularity. Your entire class hierarchy must be understood/evolved as a whole if you use implementation inheritance, that's the only way to avoid running into severe problems with its semantics as the code changes over time. Of course that's not exactly the most common approach given that class hierarchies in many real-world codebases are large and it's just not practical to survey them as a whole.