rhmb

No argument that LR may be the standard to beat. That said, On1 Photo Raw 2020 (and probably others) is hot on their heels. I migrated over to On1 earlier this year to replace LR 6.14 which was no longer supported on Mac Catalina. Overall On1 has met expectations as a replacement as DAM and for tasks formerly done with Adobe Elements.
As I understand it, On1 interfaces with AP in much the same way as does LR. On1 can send files to and from AP in psd, jpg or TIFF format but it cannot read the AP format. Photos sent to AP for editing must either be flattened to save back to On1 in a compatible format or saved separately in AP format if one wants to keep the layers. 

Hello RHMB,
I did already post (the wrong way I guess) a few hours ago, Hence this new attempt:
The "Edith with Affinity Develop" stopped working "ages" ago. (If I remember correctly, with the introduction of version 1.7). I disabled this one a time ago , also because it's limitations add nothing special to what Photo's can do in the meantime. Lots more possibilities for that matter. Of course I tried with this new version, but it simply doesn't work for RAW, as you noticed.
Glad the extension "edit in Affinity" does work for you as well. I find this one much more valuable if I want to use layer or other advanced functionality. However using this option has, to me,  one big disadvantage. If you save back your RAW-edits to Photo's, and like to continue afterwards were you left it isn't possible. When you re-open the file from Photo's, Affinity will start again wit the original RAW, without any edits. So you have to start over again, the history is gone ! Other apps like RawPower, and f.e.On1 keep the history, so you can pick-up where you left. Due to the way apps and Apple Photo's work this is limited anyway.
If you like there is a very interesting article by Nik Bhatt
https://tidbits.com/2019/06/14/the-ins-and-outs-of-non-destructive-editing-in-photos-for-mac-and-ios/
Ben

Affinity Develop 1.8.4 back to its familiar "Unable to edit with" error  ...... Oh well, maybe when 2.0 comes along.
Fingers crossed unfortunately did not help.
Some consolation can be taken from the fact that Apple Photos is actually a fairly good raw editor/developer in its own right, arguably as good as if not better than Affinity. There is really minimal if any advantage to developing raw photos with Affinity within Apple Photos, at least none that I can see. 

Disappointed, not surprised, cannot Edit In Apple Photo and Save 1.8.4.  Five Stars for longevity award.

I have the opposite experience. The Develop extension will not save edits to raw photos but the Edit in Affinity extension does - most of the time. But since the Edit in Affinity extension cannot be relied upon to save edits every time, and since the Develop extension cannot fulfil its primary purpose which is to develop raw photos, I no longer use either within Apple Photos. 

Hard to say how much the issue is with Apple and how much with Affinity. That said, the On1 Photo Raw extension works flawlessly with Apple photos as does External Editors for Photos (dates back to High Sierra days but still works with Catalina ....not bad for a $1.50 app). 

No argument that LR may be the standard to beat. That said, On1 Photo Raw 2020 (and probably others) is hot on their heels. I migrated over to On1 earlier this year to replace LR 6.14 which was no longer supported on Mac Catalina. Overall On1 has met expectations as a replacement as DAM and for tasks formerly done with Adobe Elements.
As I understand it, On1 interfaces with AP in much the same way as does LR. On1 can send files to and from AP in psd, jpg or TIFF format but it cannot read the AP format. Photos sent to AP for editing must either be flattened to save back to On1 in a compatible format or saved separately in AP format if one wants to keep the layers. 

I can’t speak to that as I do not have Photoshop, only Elements and LR. 
I have not found Catalina to be more buggy than previous versions for anything other than Affinity. The biggest change that affected me and perhaps many others was that 32 bit apps will not work on Catalina. Before upgrading users are encouraged to confirm whether any of the apps they use would be incompatible with Catalina. 
 
The two Affinity extensions which have issues with Catalina are Affinity Develop, which does not work with raw photos but does work with jpg (which don’t need developing) and Edit in Affinity, which in some circumstances for some photos does not want to save the edit. There are a number of threads on the bug forum and the desktop forum on this. The other extensions seem to work ok. 
 
I suspect part of the problem with them solving the Affinity bugs in Apple Photos is that it only seems to affect some users. 

I'm running Photos 5, the version that comes with Catalina. Using Affinity within Apple Photos has been hit and miss, sometimes the work arounds work, sometimes there are surprises. So I don't use it with Apple Photos anymore. If I need to use Affinity, I'll use it in stand alone mode or with the External Editors for Apple Photos app, an app that dates back to High Sierra times but still does the job with Catalina. 
I knew before upgrading to Catalina that LR and my older version of Adobe Elements would be problematic as 32 bit applications do not work with Catalina. Although LR 6.14 (which is 64 bit) can work in Catalina as long as it was installed before upgrading to Catalina, it is not supported by Adobe and components such as the installers and the application manager are 32 bit, so ever anything went wrong, it would have been impossible to re-install LR. My initial plan was to switch to Apple Photos for the DAM and basic edits and use Affinity Photos for more advanced edits. That worked for a couple of weeks with Mojave but fell apart with the upgrade to Catalina and the subsequent issues between Apple Photos and Affinity (unlike some, I didn't have any issues with Mojave, they started with Catalina). I then discovered On1 and it pretty much does everything I need for now as both my DAM and my main editing application. I'm not a power user so it works for me. 

With regard to Lightroom like products, I switched to On1 Photo Raw in early 2020 as a result of incompatibility between LR 6.14 and Catalina. I am very pleased with On1 Photo Raw to date. As a DAM it is very similar to LR, the migration of edits worked better than I expected, and with its layers capability it also takes care of the relatively light Photoshop-type tasks I used to do with Adobe Elements. 

Very much regret to confirm that there are still problems with Affinity Photo extensions in Apple Photos, in particular Edit in Affinity Photo and Affinity Develop. It seems unlikely there is a fix in line anytime soon, indeed, it seems to be very low on the priority. 

My apologies if my comment implied that Affinity was not at all compatible with Catalina. As a user, I was referring to the challenges some have had when using Affinity with Apple Photos, not to the overall compatibility of Affinity with the Mac OS. 
I may also have interpreted too broadly your statement that the issue with Apple Photo was fixed. Obviously you were referring to one set of issues (or just one issue); I was referring to others. It is encouraging to hear that these are being worked on. 
And on a positive note, I confirm that I have had no problems to date using Affinity both as a stand alone and in conjunction with my main photo management and editing app (which is not Apple Photo) on a Catalina Mac.