I have no experience with internal (IF) DSP radios but have had reasonably good luck with two filters... one DSP and the other analog. And I agree with AD0QK (above) that the ability to tune them is important. And both of them, oddly enough, reduce background QRN about the same. I have one on my old FT-767GX and the other on my K2.
And while they are perfect for CW they are both less than perfect for SSB. I may have to plunk down the money for an internal DSP unit but the reviews on those have been spotty, too.
The two filters I use are the Datong FL-3 (which is the FL-2 with an auto-notch feature) and the Timewave DSP 59+. Both of them are reviewed in depth here and there is a YouTube video that compares the FL-3 with a DSP filter and both of them seem to be about equal. The video is highly informative but the DSP unit it compares the FL-3 to seems to be no longer available. https://www.youtube.com/watch?v=XI2AcJCfOBo
I find that they can be fiddly to work with; especially at first. And not sure how they'd do with the rapid fire DX or contest QSOs.
Both of them, by the way, draw more current than my K2 does on receive. Lots more.