I am afraid that conclusion appears less than objective. There are many reasons why mixing manufacturer styles may not work and relative size is only one. As to size, there is little doubt that the scale of various of the lines listed above is different for different releases and the dealers so called warning, while less than completely accurate, could have simply been an appropriate caution to apprise his customers that products from these lines do have their differences, in style and accuracy, as well as size. Without more direct evidence, it would appear that the assumption of "significant customer dissatisfaction" is simply an aspersion without basis.
I am not sure how much "digging" is required to view a page on a manufacturer's website that is freely accessible but I do not find the scale discussion there anything more than informative. To suggest it was drafted by PR or legal is a bit of a pejorative leap.
What is “reality distortion” about the description quoted? It reflects a simple statement of a change in the labeling of their figures which occurred early in their history. In fact, the earlier labeling was based on a well-established convention of measuring figures from the feet to the eyes and in fact the FL figures I collect do measure 54-56 mm for that purpose. From the sole to the tip of the unadorned head (another scale convention), they measure 59-61 mm, hence the most appropriate 60mm classification. Why or how that is "reality distortion" is most puzzling.
I am also afraid that the phrase “true 1:30 products like Figarti and K&C” is an oxymoron. It, of course, assumes that those products are 1:30 scale. I do not know about Figarti but the K&C figures I have are certainly not 1:30. As other posters on this thread have noted, they measure pretty close to 1:28 -1:27, unless you are using the sole to eyes convention. So perhaps it would be more accurate to simply call them K&C scale.
Perhaps you don’t but I don’t think you speak for most collectors. Moreover, you are apparently encumbered by the false assumption that all of your collection is 1:30. Unless you have some K&C figures that I have never seen, I think you will find that your collection does already include other scales than 1:30.
I like to think I am a discerning collector and I very much appreciate the offerings I have from First Legend. Their sculpting and painting is akin to premium Russian figures for a relatively bargain price and I have found the accuracy and consistency of scale within their lines to be without equal. In the lines I collect, they do not match most of the figures from K&C and CS, not only for relative scale but also for style, as others have said. So if your interest is solely mixing them with those lines, I would suggest you are better served by staying with the lines you currently collect. I hope that is helpful.