Fake reviews are deliberately biased, and may contain false information about a product's quality and of course will ignore its flaws and defects. It's just advertising disguising itself as user experience.

It is probably very common (and as an impulse understandable) but I think it's still wrong even without trashing your opponents. Saying your product is better than it is is not much different than saying explicitly that it's better than the other guys. Either way, it a cynical lie.

Lying is lying and not disclosing links is lying by omission. It's all personal preference but my line in the sand is pretty clear. If you eat to be ignored and derided, let someone else find your bias and kickback. Be up front and you will get a reputation for it. That's my 2c.

It's against European law. But companies flouting the law doesn't bother some.