i just picked up a kickr core and it seems extremely stable. The base bars are 2" thick and either 14" / 12", or 12" / 10" wide. Maybe if you’re putting out 2k watts and wildly sprinting you could knock it over, but I don’t think there’s anything that makes it more likely than any other trainer to fall over.
There have been some issues reported with the kickr core 2018 models, @mcneese.chad Anybody have a Wahoo Kickr 2018 that DOES NOT have any issues? - #24 by mcneese.chad, I’ll try and dig it up and edit this reply but tagging him if he’s got it off hand! I picked up a Kickr core refurbished for $620 shipped direct from them, which is by far the cheapest option, of the 4 you mentioned.
Also, this is a good thread.