I believe it is useful information for the owner of the web site. Whether you like it or not, people make decisions based on seemingly irrational factors, such as the design of the web site. It is particularly regrettable in this case, as the main design idea of the site is fine, but there are some glaring errors in the implementation. It would be simple enough to fix and could make a big difference for the company.
You need to take the bare minimum ten seconds or so to go into some actual detail in order for a comment like that to be useful. This is why you also have a reply from the creator prompting you for further detail. If you're really busy, at least give some impression of your screen size. If you have more time, maybe link to a screenshot. Vague criticism alone is worthless though.
Personally I'd say that the font weights are far too low on much of the text. Thin text is in vogue and often looks great, but it's a bit overdone in this example. The phrase "Address book storage you can trust" at the top practically bleeds into its own red background. The sentence below is less readable still. And the text in the two buttons below that isn't vertically centered at all.
I believe it is useful information for the owner of the web site. Whether you like it or not, people make decisions based on seemingly irrational factors, such as the design of the web site. It is particularly regrettable in this case, as the main design idea of the site is fine, but there are some glaring errors in the implementation. It would be simple enough to fix and could make a big difference for the company.