This issue is also limited use in some social content use cases, specifically when users try to use emojis like ("pile of poo").
For a customer I am currently working on I upgraded the encoding to utf8mb4 on a MariaDB 5.5.50 install today. The main issue encountered was in relation to the key length in some Activiti tables where the key length with 4-byte unicode ended up longer than the default 767 byte index key length limit for InnoDB. This affects e.g. the table act_re_procdef which includes the columns KEY_ (varchar 255), VERSION_ (int 11), TENANT_ID_ (varchar 255) in a unique key constraint.
I dealt with this issue by using COMPRESSED row format (Barracuda storage file format) and configuring innodb_large_prefix to "on".
Note: I wanted to include the 4-byte unicode character for "piile of poo" in this comment as an example but JIRA apparently also does not suport 4-byte unicode.