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Examples include: use of globals, inefficient queries, mixed templating, lack of namespacing, doesn't follow PSR standards.

They actively refuse to use parameterised sql queries, opting for their own home baked userland implementation instead.

Probably the use of globals etc. This might also be a backwards compatibility thing though.

Developing anything for wordpress is a nightmare, it's not only PHP-nightmare, but also plenty of bad practices, poor API, wired workarounds...