I think the "issue" with a lens like this is the intended audience. A 400mm eqv. FoV lens (or 560mm with the 1.4x TC) is intended for sports and wildlife (400 is a bit short for wildlife, but 560 is suitable).
However, m43 cameras are at a disadvantage IMO when it comes to shooting sports against the likes of CaNikon with their DSLRs. The E-M1 II is the best sports camera the m43 system has to offer, and while the the AF is much better post FW 2.0, the lingering "issue" is the noise performance of m43 sensors.
When you can control your light (i.e. portraits), shoot from a tripod (i.e. landscapes), or are working in good natural light (i.e. sunny wildlife), m43 is perfectly acceptable. But it's once you get away from good light where m43 cameras begin to struggle, and cameras like the D500 or D5 really come in to their own.
When you're shooting sports, shutter speed is critical. You need to freeze the action to get a sharp shot, but if you can't control the light, that means you need to rely on your ISO to get the shutter speed up to where it needs to be. Sometimes that means shooting at 3200, 6400, 12,800, etc., and no matter how you try to slice it, m43 cameras will not compete at these higher ISO's against larger, less pixel-dense sensors.
Don't get me wrong, I love my m43 gear. I have two E-M1 II's, PRO primes and zooms, etc. Great gear. But like anything else, there's a proper tool to do the job, and when it comes to sports shooting, convincing sports professionals who put food on the table by covering these events is going to be a tall ask. I applaud Panasonic (and Olympus with their 300/4 PRO) for making this lens, but I really don't know how well it'll sell based on its intended market. Having said that, the 300/4 PRO seems to have sold very well to amateurs/enthusiasts for birding.