Should the collation of migrations be changed?

[Tobias]

I think the collation of the migrations should be changed from utf8_unicode_ci to utf8mb4_unicode_ci or utf8mb4_unicode_520_ci.
since utf8_unicode_ci has some problems.

Citing from https://make.wordpress.org/core/2015/04/02/the-utf8mb4-upgrade/ :

The difference between utf8 and utf8mb4 is that the former can only store 3 byte characters, while the latter can store 4 byte characters. In Unicode terms, utf8 can only store characters in the Basic Multilingual Plane, while utf8mb4 can store any Unicode character. This greatly expands the language usability of WordPress, especially in countries that use Han character sets. Unicode isn’t without its problems, but it’s the best option available.

utf8mb4 is 100% backwards compatible with utf8.

Here a more detailed explanation:
http://mysqlserverteam.com/sushi-beer-an-introduction-of-utf8-support-in-mysql-8-0/