Heh, tongue firmly in cheek: there's a good reason to change from I2C to SPI. I2C's pretty shit in comparison :D
More seriously, I've had interesting mixed results. I had a hobby-maybe-commercialize-next-year Christmas lighting project that I started in March (if I start in March, this will definitely be ready for Christmas!) and got 90% of the way there... and then life got busy and it's sat there untouched with the holiday season coming up again quicker than I'd like. I threw Fable at it first to assess what's done, not done, look for schematic mistakes/improvements (it rightfully pointed out that the cheap LDO I was using was near its limits and suggested a better part).
Opus did a reasonable job of swapping out the LDO, putting reasonable values on all of the pull-ups/pull-downs, etc. Some of the labels did end up in slightly weird places and needed some manual correction. It also did a decent job of floor planning on the PCB side, but pretty much refused to do any routing other than power traces.
I've installed https://github.com/drandyhaas/KiCadRoutingTools this morning after seeing this article last night... we'll see how this goes. My first manual attempt at running the router did decent but it failed a bunch of DRC spacing rules and insisted on via-in-pad to make it all work. That's, I think, a side effect of Opus's floor planning. It's interesting to play around with to see what's good and not.