pro — I've gotten to use both the Echo Show 8 (8" screen, gen 2) and the Nest Hub (7" screen, 2nd gen), and I really prefer the Echo Show 8:
even though the screen sizes are similar, the Echo Show 8 has a noticeably bigger speaker, resulting in much better bass response
pro — the wake word can be configured to a few different words, making it much easier to use multiple Alexa devices in the same room
con — Amazon explicitly discriminates against polyamorous people, because only two adults can login to an Alexa device. Let me be clear: I don't have an issue with limiting the number of people who can share an Amazon Prime account to two adults, because it makes economic sense that Amazon doesn't want to provide expensive shipping for an endless list of adults. What I do have an issue is extending that specific restriction to Amazon Echo devices. There is no reason that Amazon needs to limit the number of Amazon accounts (Prime or otherwise) that can login to an Amazon Echo device.
I understand that some Prime benefits extend to Amazon Echo devices (e.g. Amazon Prime Video, Amazon Music Prime, Prime Reading, Amazon Prime Photos). Perhaps it should be understood that multiple adults living in one place are simply going to have access to each other's subscriptions, especially cohabiting adults in close relationships, and not go to great lengths to police this. (though it may be justified to confirm those adults actually live together, say by checking their Amazon shipping addresses, or at least checking that they are only using their Amazon Echo devices from one location)
I also understand that the distinction between "adult roommates" versus "adult cohabiting polyamorous people" might be a subtle one from a business perspective, and that there might be economic incentives for Amazon to try to distinguish between the two. However, whenever it's impossible to distinguish between the two (regardless of reason, whether it be technical, operational, logistical, ...), it should be recognized that preventing cohabiting adults from sharing content because "they might be just roommates, and there are a LOT more roommates than there are cohabiting polyamorous people" results in actual harm to polyamorous people, meaning it's a reputational risk for Amazon. (or it will eventually be, anyway, once discrimination against polyamorous people becomes a more widely known issue)
con — The options for Alexa in your vehicle are limited.
I already carry an Android smartphone everywhere, it connects to my car's Bluetooth just fine, and that seems like a much better option than buying an Echo Auto.
If I really wanted to, I could configure my Android to use Alexa by default instead of Google Assistant, which means there's no need to buy new hardware. But I'm not sure if I'd ever do that — for in-vehicle use, Google Assistant seems to get much better reviews than Alexa.
con — I've been able to use both the Echo Show 8 (gen 2) and the Nest Hub (7", 2nd gen), and I prefer the Echo Show 8. (see above)
con — It is not possible to change the wake word to something other than "OK Google" or "Hey Google". This means that, when you have your phone in the same room as your Google Nest device, it's going to be difficult to indicate which device you want to respond to your command with.
It's true that Google Assistant has "deconfliction" functionality — when it detects that multiple devices have heard the same voice command, they will do an automatic negotiation to ensure that only one of them responds. However, it's somewhat random which device decides to respond.
It's possible to configure the wake-word sensitivity on a per-device basis, so it might be possible to built in a slight preference for one device over another. (TODO — try this and see if it works for me)
It's also true that you can say something like "Hey Google, play Lizzo on YouTube Music on the Nest Hub". However, those voice commands start to get kind of long. I've also been able to use multiple Alexa devices in the same room, and being able to use different wake words to distinguish between Alexa devices just feels much easier than on Google Echo devices.
pro — Google does not limit the number of accounts that can login to one Google Nest device (see discussion of "discrimination against polyamorous people" above)
pro — for in-vehicle use, IMHO Google Assistant is a better option than Alexa (see above).
I've never bought an Apple device, and I'm not sure when I intend to start.
In general I prefer having more control over my devices than less (preferring manual transmissions and dual-clutch transmissions, for instance, and preferring gargantuan settings panels), so I'm not sure that Apple's approach of providing a slick user-experience but limited ways to dig around under the hood would resonate with me.
My overall conclusions
Each of these voice assistants feel like they're a vast ecosystem, tying together many different services (both manufacturer-provided as well as third-party integrations). IMHO, this makes it much more complex to attempt an apples-to-apples comparison. I've decided that it's better to simply try each voice assistant out in turn, and figure out what I personally think about the specific services that I might use on each platform.