Silver Efex Pro and Viveza got a major interface overall and a couple new features along with better U-Point control which is the primary reason to get these plugins - in my opinion. I really like the new look. It's modern, easy to read and better organized. Viveza has needed some love for a long time and it is finally where it need to be. Silver Efex pro got the addition of Clearview, borrowed from DXO Photolab which clears out haze and can really add drama to a dull photo. I already have that in DXO Photolab thought and adding more presets which was a large % of the update to me is cheating when listing new features.
To make this worth the upgrade price, they really need to apply the new interface to all the plugins and maybe they will before NIK 5 is released. If not I won't be updating again as while the new look is great, they all work fine as it is and they would have to add really amazing new and unthought of functionality to make me spend more money. They update DXO Photolab quite a bit between paid iterations and I hope NIK will get some of that attention too.
That said, if you don't have the NIK collection at all, it is well worth having and does amazing things. It integrates more completely with Photoshop and Lightroom but it works nicely with Affinity with the exception of Perspective Efex which you have to run as a stand alone. HDR Efex is my favourite HDR processor but to use it with more than one photo, which is what HDR is really all about, you have to either do the initial processing in Affinity and then open HDR Efex or open is as a standalone to import a series of bracketed pictures at once which is what I would recommend.