My understanding is that, if fitted correctly, the Pro is very safe, but if not well fitted it obviously isn't safe. As would be the case with other car seats, but with the Pro the fitting is difficult and that's why it ended up with its don't buy warning.
I think this is Which's interpretation of it, yep. They've done this before for other lower-tether ERF seats, so it's a bit of an old story for a lot of the ERF specialists. As you say, if you're willing to put a bit more time/effort into understanding how to fit a seat and ensure it's done properly, then a low score in ease of use isn't necessarily a dealbreaker.
However the OP's and my and several other posters' concern wasn't with the Which warning but specifically with the test results straight from ADAC, which is the German version of AA, they do all the testing on child seats that then gets sold to the various European consumer publications like Which? in the UK and Stiftung Warentest in Germany.
If you ever want to you can go to ADAC's website (which is in German) and look up the results directly. They publish the results for each part of the testing - they look at crash safety, ease of use as said, and a couple of other things like ergonomics and the presence of pollutants in the fabric (which is a bigger concern in Germany than it is in the UK, Which? don't use these results). And if you want it broken down even more, you can look at OÄMTC, which is the Austrian version of AA, as they publish a free numerical score for each subcategory so you can even see the different scores e.g. for frontal crash test vs side impact crash test.
In the past, any seat which has passed the strict Swedish "Plus Test" has got a very good or higher result on ADAC for the crash test, especially the frontal crash test. But this time the Axkid Minikid Pro got a frontal crash test score of 3.2 which is almost unheard of. Previously, plus-tested seats' scores for safety have been between 1.2-1.9 (lower is better) and the frontal crash test scores are even better than the overall safety score - some are even below 1 (which is excellent and rarely seen).
3.2 as a score to compare is more typical of how cheap, generic (e.g. Nania) forward facing seats score on frontal crash tests. It's not a complete fail or anything, those tend to be 4 or higher, but it's bad - indicating higher than average risk of injury compared with other seats on the market. It's really surprisingly bad for any rear facing seat but especially so for a plus tested one.