Software
There's no kind way of putting this: Huawei's software sucks. They use an unnecessarily overbearing skin that regresses from stock Android in several ways, and the actually useful additions Huawei makes don't justify sweeping changes to the visual style and functionality of Android.
On top of this, Huawei has a poor record with software updates. With a phone like the Google Pixel you can expect monthly security patches and the fastest updates to the latest versions of Android, including a developer preview program. With the Mate 10, you'll be lucky to see the latest version unless you upgrade to a new device, and their security update track record isn't great.
One of the most frustrating things about Huawei's EMUI software is the way they adjust notifications to be less information dense. On my Pixel 2 XL with default font sizes, a ton of information is fit into the notification pane, but with the Mate 10, everything is padded out and increased in size for no good reason. Additionally, some functions such as quick replies didn't work in some situations, which is bizarre when the feature works so well on other Android devices.
The visual style feels dated compared to what Google provides on the Pixel with stock Android 8.0, and some stock apps feel cluttered with elements and less usable than their counterparts on Google's default offering. Plus, I had to turn the 'view mode' down to small on the Mate 10 to increase information density and feel like I was making the most of the larger display, whereas the equivalent of this viewing mode is the default setting on the Pixel 2 XL. The Mate 10's default density seems designed for someone with poor eyesight, rather than thought to the larger display on the handset.
The settings screen is condensed far more than other offerings, but not in a good way: many key features like the date and time are hidden under the non-descriptive 'system' submenu, while other areas that feel like "system" settings such as the storage menu get their own top menu. Using such generic labels can hurt navigation.
The Mate 10 also includes an egregious amount of applications out of the box, most of which is bloatware. The 'tools' folder is filled with things like weather, health and backup apps, which most people will eschew for their favorite third-party Play Store apps, making their inclusion (and development) completely unnecessary.
Pre-installed junk also includes a lot of duplicate apps, like a second gallery, email client and music app, so you will see the dreaded 'choose what app to use' pop-up dialogs at times. Then there's just straight up bloatware like CNN and Booking.com.
Huawei needs to completely overhaul EMUI if their software is to remain relevant up against a fast-evolving stock Android. Or they could switch to stock Android, but I can't see that happening.