They can occasionally be adequate, but yeah mostly they are not so good.
Packt's niche is not paying authors much, not editing or checking their work much, but letting just about anyone sign up to write a book on their pet topic (often it seems as a loss leading personal or project marketing exercise).
The advantages of this is that they get a wide coverage of topics and are often the only publisher who will produce a book for less trendy or more obscure/specialist subjects. The downside is that the books on average are worse than everyone else's.
Rapidly aging tech books on niche topics will never sell many copies, so only the very cheapest to produce will get published.
Quantity over quality. Sometimes though for a less popular and badly documented open source project, a Packt book might be the only option.