Good to know, this probably explained why the notionally transparent artwork was imported with a white background. But the fact that it’s not a true .ai import I don't think this is ever going to be a concern to me because I never use SVG to typeset flowing blocks of text. The main things I'm concerned about are:

1) Path and style integrity

2) Artboard dimensional integrity

3) Export file quality

In my testing, I was able to open a file I had edited and saved in Illustrator, export it as an SVG, it was an utterly pixel-perfect drop-in replacement for a previously exported SVG. I was also able to confirm that all of the SVG files I care about can be opened directly, edited and exported with no errors. That's enough for me to confidently say goodbye to Illustrator forever. It's robust enough that I know I can deal with the occasional import glitch if it ever does happen.

There are some really annoying things about Designer, though - like how it insists on rasterizing Gaussian blurs, even though SVG supports them as a vector operation...

affinity artboard support is newish, and not great. you may want to deep dive into it to be sure their implementation is good for you

Yes, I did. My files are simple. Around 15 or so assets. One file per asset, no elaborate effects used.