The Plzen issue:
- Several small carnivores in shoeboxes (genets & slender mongoose come to mind).
I feel quite surprised myself to vote for Plzen with their atrocious enclosures for hausa genet, slender mongoose and ring-tailed vontsira.
You definitely won't get any disagreement from me about the Slender Mongoose exhibit, barring any recent changes neither of us are aware of; however I think you are somewhat exaggerating how small the exhibit for Hausa Genet is, and I'd definitely not call it "atrocious":
We've differed on the vontsira exhibit in the past, partially because you disbelieve my assertion that they do have outdoor access
Pairi Daiza has some pretty good enclosures too. Their puma enclosure is far superior to the cat enclosures in Plzen and the expanded snow leopard exhibit is quite attractive, as is the cheetah exhibit. I suppose the binturong are also fine in their weird mix with bharal, but they have trees. I am just not a fan of most others. The Steller's sea lion exhibit is great at first sight, but the actual swimming room is quite limited with all the rock work. And there are faults to be found in most other exhibits as well.
Another example which comes to mind is the fact that when their last giant otter died and their exhibit - which for a long time was one of the very worst enclosures at the zoo full stop, let alone for a category species - became vacant, they promptly shoehorned in Smooth-coated Otter, for which it is only *marginally* better suited rather than spending any time, thought or some of their plentiful money to either a) obtain a species for which the exhibit would be appropriate or b) redevelop the exhibit whatsoever.
As such, they now have multiple Smooth-coated Otter in what is still one of the worst enclosures at the zoo
On the subject of the Steller's Sealion - I believe that the same applies to both the walrus and polar bear exhibits, doubtless due to the same reasons as those I mentioned to @Lafone above.... which all once again sums up one of the major issues with Pairi Daiza; they have access to a ridiculous amount of money with which to build excellent and world-class exhibits, but deliberately choose to build exhibits which are limited in size, flash-over-substance, often provide the inhabitants with scanty area to escape from view, and which (above all else) feel like they are specifically designed to maximise the chance of making an even more ridiculous amount of money.
Or as @amur leopard put it - Pairi Daiza is "a zoo which very evidently has the means to make brilliant living situations for their animals and yet, again and again, manages to avoid doing so."
Leads me to ask people who have visited - is it hard to find your way around PD or is the navigation and signposting sufficient to identify the animals you want to see? I do factor this into a rating as in order to appreciate a thing you have to be able to find it.
It can be pretty confusing, yeah - last time I visited there were also periods of time during the day when they temporarily closed-off paths or made them one-way only, which rather added to the confusion!
overall the initatives of Plzen are not comparable to the size and variety of projets at Pairi Daiza Foundation with multiple of them focussing on carnivores.
Given the fact that the bulk of their "conservation" spending on carnivores comprises outright greenwashing - the massive amount of money paid to China each year for their Giant Pandas - I would contend this is less impressive that you claim!
If i remember it correctly they already had five American black bears. And as this wasn't enough they made an unnecessary import of ten animals from America to put them on display at the same exhibit with the already present bears. @lintworm already mentioned the result of this.
And for the reference of those unfamiliar, here is the exhibit in question - it would be a very good exhibit for two or three bears, but it is far too small for over a dozen bears! Moreover, as I alluded above, the visitor path surrounds the exhibit on all sides, meaning the only way the bears can escape view is to go indoors:

