Er, no, that would be locked-down GrapheneOS, on a Pixel.
There's a multitude of reasons - but here's the biggest one: Apple's Lockdown mode is all or nothing. You can't selectively enable certain features that you may truly depend upon. On the other hand, GrapheneOS allows you to selectively disable individual security features that may be too overbearing. It would be far easier to daily drive a GrapheneOS Pixel than an iPhone in Lockdown, for that reason alone.
My threat model includes a few things, but one of them is that I don't want my data available to advertisers.
GrapheneOS sort of supports that, but I found that it's nearly useless as a daily driver when set up that way. Even with Google Play Services installed in a sandbox, GPS stuff breaks, the camera is flaky, and third party apps don't work reliably. Also, the remaining built-in apps have huge gaps (no backup, no synchronized notes, etc).
Worse, I'm not convinced that sandboxing it helps privacy that much. Without it installed, the phone had multi-day battery life. With it, it dropped to whatever Google was advertising (30 hours?).
Anyway, iOS without lockdown seems to be much more secure (by my criteria) than my GrapheneOS Pixel phone was in practice. Also, I can use all the apps that are essentially mandatory around here.
I haven't experienced things breaking, but it is slightly slower (very slow if indoors) to get a GPS lock, because by default even location requests through Google's API are re-routed to the system service, which uses standard GPS/SUPL/PSDS rather than hoovered-up Wi-Fi SSIDs. You can optionally enable Google's location service if you want faster results.
I'm not sure, all of the recent Pixels were on the Cellebrite leak list as accessible without brute-force even while cold. Of course, the recent iPhones were too. Maybe there is no solution, or maybe Cellebrite is lying a little bit with their ads.
Cellebrite has a separate category for GrapheneOS and the most recent leaks indicated up-to-date GrapheneOS was invulnerable to all attacks, whereas the latest iPhones were vulnerable to AFU attacks.
I'm genuinely curious - what issues have you run into with Lockdown? I've been using it enabled for the times I have been carrying an iOS device (vs my preferred flip phone), and I've yet to run into anything I consider a deal breaker.
I can't get animated gifs in MMS/texting threads. Oh darn. Doesn't bother me, they're usually content free fluff anyway.
WebGL being disabled means I can't use that one guy's awesome website on my phone - except, if I want to, I can disable Lockdown on a per-site basis for trusted sites (which then allows those things to work again).
... I can't get Facetime calls from random numbers? That's never been a problem for me one way or another, and, good.
I do occasionally run into websites that use some image format that doesn't render, and if I really care, I can disable Lockdown on the per-site basis there too, but I usually don't bother.
I'm just curious as to what the actual issues you've found with it are. I turned it on in a beta and haven't found any reason to turn it off since then.
The reason I use an iPhone instead of a Graphene device is that Graphene does not support sufficient device attestation to run Microsoft MDM, which means I can't get to my work calendar.
There's a multitude of reasons - but here's the biggest one: Apple's Lockdown mode is all or nothing. You can't selectively enable certain features that you may truly depend upon. On the other hand, GrapheneOS allows you to selectively disable individual security features that may be too overbearing. It would be far easier to daily drive a GrapheneOS Pixel than an iPhone in Lockdown, for that reason alone.