The main elements you have mentioned, the defects or weaknesses may be:

1. @include MQ(M):

• Problem: If you're only using the carousel on desktop, it won't provide a better experience on smaller devices. Mobile users may be deprived of some interactive features.
• Solution: It would be better to make the carousel responsive on mobile devices as well so that all users get a better experience.

1. @include themify($themes):

• Problem: Theme switching can cause problems in some browsers or some older browsers if the CSS for the theme is not written properly. Also, if this code is not associated with theme variables, inconsistency may occur.
• Solution: It is important to create a balanced and well-tested style for each theme so that users don't face any obstacles in switching themes.

1. transform: translateX(var(--pinned));:

• Problem: If the CSS variable for carousel does not work properly or if the value of `--pinned' is not given correctly, the problem with the movement of the carousel may arise. This can cause the carousel to appear in the wrong place or not work properly.
• FIX: CSS variables should always be given a default value so that the carousel can be positioned correctly if the variable's value is not available at some point.

1. Updated/Targeted Styles:

• Problem: When new styles override old styles, it's important to make sure the specificity of the CSS is correct in each case. Sometimes overriding styles can create a conflict with the rest of the code.
• Solution: Better understand CSS specificity and arrange styles in a systematic way, and avoid unnecessary use of `!important' everywhere.

These problems can arise when the code is not tested extensively or in different browsers.