Some of these bars do have spring plungers rather than rigid adjusters. All spring dependent bars have advantages and disadvantages. WP maybe tipped the balance towards the advantages, but didn't eliminate the disadvantages.
I bought the WP bar to check it out, as I have miter bar ideas I'm looking to market, and looking for the most cost-effective solution for sleds I'm looking to market. I have to say i was mostly unimpressed with the WP bars' performance, though the engineering and fit and finish is impressive. The problem is at the beginning of a cut you have limited bar engagement, and I found I didn't have a reliable wobble-free glide until three of the springs were engaged (it's pathetic with only one). After three it's silky smooth and true. So if you want to cross cut maybe 18" wide or narrower on your 24" capable sled it works great. Then again so would carefully sized BB or plastic runners for about $1 or 3 each . The difficult thing to achieve with a sled runner is consistant travel with only a few inches of sled runner engaged. WP made this bar initially for their Miter gauge product (the one that doesn't rotate). I think it's overpriced and underperforming for a sled. With a sled you don't need steel's rigidity.
I've done some experimentation in shop-producing a bar with a similar mechanism. (I was working on these before I became aware of WP's version!). This video shows the idea: https://www.youtube.com/watch?v=-_EPwn8GIhY
(Please try to ignore my obnoxious tone! Badly in need of media training...) My latest versions are machined integrally from BB on a CNC rather than laminated, though they can fairly quickly be made with a drill press and router table too. Once attached to a sled they work as well as WP's $70 bar, and can be made as long as you want.
With any spring dependent bar, there's an optimal amount of spring you want and an optimal fit, so the bar really should be a close fit without the springs. WP's claim that their bar will work well out of the box on all slots is simply bogus. The WP's bar works abysmally on my Dewalt job site saw with it's nearly .77" slots. I made a wider barred version of my wooden spring bar that works much better there.
I think their marketing is so slick they succeed at making buyers fall in love with their tools before they've even touched them, and then they look and feel so good in your hand it's hard for many people to notice they perform no better than a mediocre version of a similar tool. I paid all this money for this tool---i better love it!! At least that's my experience with this bar. I was ready to be convinced of the bar's superiority. I wasn't. To be fair, I read that one user who complained of sloppy performance was sent stronger springs that made it work better. One-size-fits-all simply doesn't make sense for a spring dependent miter bar.