@if Rule Units

The @if rule in SCSS cannot accept <, <=, >, and >= if the value has percent unit.

These declarations are accepted:

== with % unit:

.myClass {
    margin: 0 auto;
    
    @if ($wcfLayoutMaxWidth == 90%) {
        padding: 50px;
    }
}

<, <=, >, and >= with px unit:

.myClass {
    margin: 0 auto;
    
    @if ($wcfLayoutMaxWidth >= 1800px) {
        padding: 50px;
    }
}

However, the code below will result to an error:

The SCSS code contains an error: Could not compile SCSS: Incompatible units % and px.: line: 201, column: 2.

<, <=, >, and >= with % unit:

.myClass {
    margin: 0 auto;
    
    @if ($wcfLayoutMaxWidth >= 90%) {
        padding: 50px;
    }
}

Thanks a lot for the explanation and the link.

It's used before in 5.2 with no errors. I think this can be mark as Not a bug if this is the effect of the updated SCSS compiler.

I realized now that my logic is really faulty. I did not take into consideration that there are pt, rem, em and some advanced user may even override it with vw.

Tim Düsterhus solution is the way to go.