The only real gotcha i've noticed when going from SQL server 2008 to SQL server 2012 or higher is the identity\sequence issue. If you have columns that use an identity value, these values jump upon a sql server restart. Now while it shouldnt really be an issue, it has upset some people 😀

There is a trace flag to force the legacy (sql 2008 and lower) behaviour.

Apart from that, do you plan to upgrade the OS too? Upgrading the cluster is feasible if you just want to upgrade the sql server version but moving to new hardware and OS as part of the upgrade should be considered.

What is the state of your current hardware, how much service life does it have left?