The round wobble board might even be better than the Slackblock depending on what you’re after. Both really involve the ankles much more than a basic balance board but the wobble board might be more 360 than just side to side. I’m not a physio of any sort so I don’t know if that’s good but I have weak ankles and wobbling around on them on some sort of unstable platform SEEMS to my uneducated self that that might be good 
My understanding is the Slackblock was originally made to help train for slacklining and if you scroll down on the webpage the company sells an indoor slack line set up. Its really unstable side to side but not so much fore and aft but you can stand on various different ways to get different planes of movement. I like it because for the minute or so I can stand on it without dabbing a foot I imagine myself slackline across a 1000" drop