Is there any reason to use = instead of <=> in MariaDB/MySQL? It seems that the = operator is only desirable if the null is desirable for result of the expression.
Are there any consequences of replacing every = with <=>? Even if both operands can never be null (for which the behaviour should remain the exact same)?
First of all, the meaning of NULL is quite unclear. There are many articles in the Internet about how people interpret it.
Therefore, using <=> could just add to the confusion and you risk than other developers may not understand it well. It strongly deviates from the standard behavior most people expect from queries. I would suggest you avoid it unless there are strong reasons why to use it.