I've always been intrigued by the different actions and what people say about them. It really is true that you need to try them for yourself if you can, but I thought I would write my experience in case it helps anyone else as I know some still have to buy blind if you don't have a dealer that stocks Kawai or Casio near you.

CASIO

I managed to find a shop in the UK near me where I could demo the Casio AP-S450 with the new Smart Hybrid action. This is the action which features in the new for 2024 Casio Celviano AP series. The S450 is particularly appealing being super shallow in terms of cabinet design, so it can go into small spaces.

Something that immediately became a problem which I just would not have thought about.... the pedals. They're like...tiny. :) Well apparently they are not "full size". I found playing in shoes that I was always accidentally pressing the damper and middle pedal together. It was most off putting. However, I gave it a fair play for a while to feel the action after being told it was similar to the Casio PX-S7000. I would say yes, it's similar, but different enough to be easily noticeable. The S450 feels nicer. It feels more solid. Different. Higher quality.
What I would say though is that the S450 Smart Hybrid action felt kind of strange. It felt lighter and responsive, but not too light. The main issues I have with it are that you can definitely feel the shorter pivot length in that the black keys are harder to press, and both white and black keys are noticeable harder to press near the fallboard. It also feels very plastic and rattles, especially on key lift off. It tends to rebound and knock a few times on fast let off. This is particularly apparent in the higher octaves where you can noticeably feel a notchiness as you play.

One thing that really helped me determine between this action and others was trying to play slowly and quietly. It's harder on this action. It felt less controllable. It has a noticeable bump/resistance just before fall travel of the key, which I mistook iniitially for a deliberate escapement feature, but I understand these do not feature escapement? Either way, it felt unfortunately just cheap. Lovely looking cabinet design and the cool light bar feature is nice, but not something I can consider personally. These were the only casios I was considering in my budget and cabinet design/size so moved on....

YAMAHA

I tried every Digital Yamaha in the two stores I went to and I came to a fairly simple conclusion. I don't like their actions. I currently own a Yamaha YDP-S51 with the GH action. I played the latest version of this in the YDP-S55 and the YDP-165 and it felt quite nice. Nicer than my piano as it is worn, and the keys are noisy now. The newer synthetic ivory keytops and GH3 action feel quite good, if a little heavy. Moving onto literally any Clavinova though, even the highest end ones and the key action just gets worse in my opinion! Despite costing a lot more. Heavy, notchy, snappy... difficult to describe. Less dynamic control. Overall just feels heavy and I even say that as someone owning a current Yamaha which is already supposedly heavy.

KAWAI

Tried a variety of Kawai digital pianos mainly switching between CA (Grand Feel) and CN series (RH3 Responsive Hammer 3). I was very impressed with the RH3 action in for example the CN201/301. Again, strangely if anything I preferred it over the CA series with the longer pivot. The CA series felt markedly better than the Yamaha to me but still a little heavier than the RH3. Both nice in different ways. I could be happy with either. The RH3 felt a little cheaper and plasticky but a really good general key action. I think I prefer it to my current Yamaha GH one.

ROLAND

I tried the PH4 action and this was a big surprise to me as to how much worse it felt compared to the PHA50. I read a lot of things online where people say the PHA50 is an extension of the PH4 and just a little improved. Well to me the PH4 when I got down to play it having played the PHA50, felt massively inferior and just nasty to play, similar more to a heavier, snappier action like the Yamaha Clavinovas. Each time I tried it, it was the quickest action I stood up form the piano on and moved away to the PHA50, because if the PHA50 is there...why wouldn't you get that. It's infinitely better.

And speaking of the PHA50: for me this is the best overall action I've felt in a digital piano. I just keep coming back to it. It seems the most balanced to me. I could play with the most control and dynamic range. It felt the nicest to play. I was more accurate with it. It felt heavy enough but still responsive enough. Hard to explain. Just a really good feel. I was surprised by this. The keys just feel tight/solid. They are certainly louder than say the RH3 action in the Kawai CN201, but that's not a huge deal. The PHA50 is still quieter than my Yamaha S51 worn GH action. You could feel the longer pivot as well. It's just the one I kept going back to in the shop. Nothing to do with the cabinet as I tried it in various Rolands. All felt good.

TL:DR = Roland PHA50 I found the best.