I think what they mean is that Cosatto seats have previously had not great scores on ADAC tests.
From memory the ones which have been tested are:
Cosatto Come & Go i-Rotate (tested as Heyner Multifix Twist)
Cosatto Hug isofix
Cosatto Hold
Cosatto Den (tested as Foppapedretti Uniko i-size)
Cosatto Ninja is the booster-only version of the harness+booster seat Heyner Multi Protect Aero, which has been tested.
The reason some of them appear under different names is because Cosatto do not design their own seats, they buy from a B2B car seat factory and put their fabrics on. All the safety testing and so on is done by the B2B company. You can tell that it's the same model under the covers because they have the same ECE approval number, meaning that they perform identically.
The Hold did well in testing, but that doesn't tell you much as it's an infant carrier and most do.
The Come & Go and Multi Protect Aero each got a "Satisfactory" safety rating which is OK but not great, the Hug Isofix was "Sufficient" (ie, just about passing) and the Den completely failed and flew off the seat base. Cosatto continued to sell it! You can buy it right now on amazon. Luckily it was never very popular because it had a weird height limit.
Combined with the fact that all-stages seats generally get mediocre scores (Sufficient or satisfactory) - only ONE out of the 16 models they have ever tested since 2002 got a "good" safety score. It's quite likely that all the other Cosatto all-stages models will be mediocre as well.
That's not to say they are awful. I think people get a bit of a skewed view if you spend a lot of time on the ERF car seat pages, because yeah, if you're comparing average to the gold standard, top performing Swedish plus tested models, then the average ones are going to look shit. But that doesn't mean that everything below that standard is equal. These seats for example, they rear face up to 18kg/105cm, they are isofix + support leg, they have really nice deep side wings, the instructions are alright (they aren't garbled or badly translated) and they even have the side impact pop out things. I honestly don't think Cosatto are the worst out there by quite a long way. They are probably about on a par with the more basic Joie seats, and people on those car seat pages LOVE Joie (while hating Cosatto). It doesn't make logical sense!
It's all about the perspective you're looking from really. If you go back 10 years these seats would have been better than the majority of options on the market. If you have £300 to spend on a seat today, I wouldn't direct you to Cosatto, because there are far, far better options for £300, or equivalent options for much less, but they aren't terrible seats. I would much rather see someone using a Cosatto seat rear facing than the flimsy forward facing Disney seats.