I'd say it depends. I was using both Nintex and K2 before and can say that Flow does roughly the same. In the modern world, the differences between all of them is a number of features (adaptors). It would be good to see what's 'complex' workflow is in your understanding. It all starts with a simple POC and will be clear during the process.
Generally, in the highly integrated environment, like O365+SharePoint+Outlook+Azure it works the best. Some tasks, like approval processes, integrations with 3rd party components, custom coding are done quicker... maybe this is just the matter of habit though :)