Upgrade SQL 2008R2 to SQL 2016 on windows server 2012R2
Well, once you upgrade there is no going back. So that's a big danger in doing in place upgrades and why so many people don't do it. Some straight up won't do it for production. In place also has more downtime. 

Unless you're on a modern OS (2016 or 2019) I wouldn't do an in-place upgrade as it limits your supportability given WS 2012R2 ended mainstream support last year.