Why did we want to leave? Honestly, it was a combination of things starting with the hateful and broken App Engine platform (API, admin, everything) to the usual Google nonsense of cancellations, adding useless features, bad prioritization of bugs, lack of fulfilling SLAs, etc whether it was GCE or another service. After working with 2 startups that were based heavily on Google's cloud, I can't recommend them unless something dramatic has changed. I can go into more details in private and a few publicly but it would be a huge post so I'll mention only a few things here.
Suffice to say, I found GCE to be anything but friendly compared to Amazon at all levels. While Amazon has a confusing and sometimes horrible admin panel, it's been my experience as an AWS customer for a long long time that it just works most of the time. Most of the time sounds bad, but it is better than "rarely" or "never" we experienced with Google (more later). When something is broken, Amazon generally fixes it or even a primitive retry will suffice. If something was really broken, Amazon gave us free credit, while Google either lied or hardly budged on bills (even funnier since Google was one of our investors).
A related and huge problem we had was that Google tended to make platforms, libraries, features, and services very 1/2 baked. Google is perhaps good at ideas and maybe even better than Amazon at building a solid core, but there is absolutely no follow-through. Too many times we encountered libraries and services that were terminated or overhauled because something was fundamentally broken. Worse, we pointed out big problems months or years in advance along with other customers and the community but saw no action. Google's dev team would continue to pretend nothing is wrong until either terminating or going back to the drawing board. I've never experienced a large company burying its head in the sand as much as Google. I love Google search, but I refuse to use any of their cloud products with the attitude that I've seen top to bottom, from sales to dev.
Another very important point I think a lot of inexperience people neglect - AWS has great support when it comes to libraries and integrations. Pick x language and generally you can find at least a working version of a library for whatever service you are using, including new features. Google's cloud services in my experience are filled with broken, slow, horrible libraries, including the official ones. As an example, there was a really bad (probably still open) form encoding bug in App Engine with multi-part forms that's been broken since almost the start of App Engine (basically, anything non-Ascii got corrupted). It's never really been fixed despite multiple people giving Google tons of data, patches, and workaround. We even got on the phone with them about it several times. GCE has honestly not been as bad as App Engine in terms of fixing things, but it also lacks features and library integrations.
As an example of a more specific GCE problem, anything that has issues with multi-cast had to have tons of patches. There was a special adapter for ElasticSearch when we deployed it on GCE that constantly had to be updated, and if you read about it, you'll see it's mainly a GCE thing. Pretty much every service, tech, or library we used that was more serious had similar issues - there was always a workaround or feature you had to manually implement that you didn't realize you took for granted on Amazon. The deployment story just given the tools was always far better on Amazon for example, but that may have changed.
Then there's installing and developing against GCE. Most of GCE for the past few years was command line only. That's fine if your command line is well written and well-documented, but GCE is most certainly not. The command line didn't even really work on Windows and not even really with something like cygwin. You can laugh, but at my other company we did a lot of DirectX and Xbox development, and developing on other platforms didn't make that much sense. Unfortunately some people in our company decided not using AWS would be cool. Developing against GCE once you got things working was no treat and we felt we often had to write tons of new libraries to do basic things that existed somewhere in the Amazon stack.
As far as the documentation, Google's services are usually out-of-date and full of mistakes. You could argue the docs are better than what Amazon has, however Amazon has the benefit of the community at this point that generally can answer your questions quickly. Moreover, Google in its own documentation often brags how good their docs are, while simple things like broken links or deprecated features are usually all over the place. How hard is it to run a broken link checker at least? The community is a huge topic, but I'll add that support was so bad at some points that the official line was to ask on StackOverflow instead (fine, but you're a huge company with a tiny install base at the time, wtf).
GCE did (does) have an admin console in addition to the command line, but the early versions were very limited. The last GCE console I worked with was in-step with most Google admin consoles looks like an ASP 2.0 app. Most things in the admin console are far inferior to the command line and full of bugs. Even finding the right settings in the admin console are a complete nightmare and sometimes require you to open other Google Services that you would think have no impact on GCE, App Engine, or anything else, and yet do. They often re-arrange how all of this works, but then leave parts of the old sites still running, so you end up with crazy nesting, conflicting options and navigation, and randomness. Mostly you feel like some dev somewhere kept saying, "I don't feel like doing this, so I'll put this all here because I can." It all looks like a bad assignment done last minute in a computer lab for a comp-sci class.
I know I seem a bit harsh, but I really wanted to try something new besides AWS the last few years. I joined 2 companies that did exactly that (independent of me), and that ended up terrible for both. There really was not much I couldn't do on AWS that GCE could do, while AWS could do plenty of things that GCE could not do at the time. AWS was far more reliable, stable, and well-supported for us as well, along with my own startup, and my past companies. Given I was a customer of Google's for a long, long time along with Amazon's and little changed with Google, I doubt it is much different now. Maybe someone can correct me, but usually when we get into these kinds of debates, the person with high opinions about GCE and the Google Cloud in general has not been on it long and certainly has not hosted a serious environment on it. I find Amazon as a company somewhat repulsive like Google in various ways, but AWS for all its faults is definitely more popular for a reason still. I hope Google can push Amazon to make their platform better, but from what I have seen the last few years, GCE is half-baked and only suitable if you're throwing a few small or personal servers in the cloud without significant interactions between them.