Got both a couple of 591’s and a 1st generation (9mm only) 561.
I bought the 561 as I had a carbon frame and QR forks and didnt’t want to risk clamping the frame. In hindsight this was silly as the frame is a C456 so built like a tank and that frame now has a 20mm bolt through (seemed fitting for the way I ride the damn thing) so the 561 stays in the shed.
I am not sure how the newer fork mounts work but make sure they cover all axle types and it’s easy to change. I wouldnt use them on forks with 9mm carbon drop outs and would even worry about old style bonded aluminium/carbon forks.
The 591s weaknesses are the wheel trays that are thin and go brittle and the wheel straps are okay but not great. The metal clips on them also rust. The clamp is okay but more suited to round tube bikes. The 598 does look like it has addressed those issues so I would go for it.
Bolt breaking should be addressed now, they did have a recall. If going on a long trip I have secured the bikes with a ratchet strap too just as a precaution to secure everything to the roof bars but this was more due to me using an old 1980s/90s roof rack at the time which was fine but not the Thule I was used to.
Some form of extra lock would be a good idea if leaving the car but I generally don’t move more than 30 seconds from the car with the bikes on.