+= has a confusing interpretation when the lvalue is a metatable invocation or function result. For readability given the flexibility of Lua's operator overloading, it is always better to be explicit.

The ++ increment operator is even worse, because it leads to statements within statements. Even an experienced C programmer might have trouble understanding what happens when I write `y = x + x++;`. Lua's grammar achieves a simple parse with no statement delimiters because it enforces a distinction between expressions and statements.