With ground schools, one gets what one pays for. There are very good schools in both the UK & Australia, and there are also do-it-on-the-cheap options.
In terms of knowledge base, good textbooks are available in both countries for those who wish to buy them. Both countries also have good resources for those who just want to pass the exams with minimal fuss.
Having done the EASA ATPL exams, I would say the syllabus is a mile wide (very broad range of topics) but only an inch deep - not much actual understanding required.
The Australian ATPL exams, which I've also done, are more like an inch wide (very narrow focus on out-of-date laws and aircraft types) but a mile deep in terms of understanding required.
Both systems provide a reasonable output of safe & proficient pilots.