1. HomePod can't pair with Android phones
In order to set up your new HomePod, you need to have an iOS device. It doesn't necessarily need to be an iPhone — iPads and iPod touches work, too — but it will not work with Android devices whatsoever, even as a typical Bluetooth speaker.
Here are the devices HomePod is compatible with:
iPhone:
iPhone 5s through iPhone X (including iPhone SE)
iPad:
12.9-inch iPad Pro
10.5-inch iPad Pro
9.7-inch iPad Pro
iPad (5th generation)
iPad Air and iPad Air 2
iPad mini 2,3,4
iPod touch:
iPod touch (6th generation)
2. HomePod doesn't recognize different people's voices
While HomePod will answer to anyone's commands, it isn't capable of recognizing individual voices. This means you can't set up user profiles or tailor the device to different members of a household.
3. HomePod can't check your calendar
The version of Siri that lives inside HomePod isn't quite the same as Siri on your iPhone. It can't check your calendar for events or make a new one.
For comparison, both Google Home and Amazon Echo have those skills — and not just for Google Calendar. Even the Echo can sync with your iCloud calendar, so it's a bit strange that that feature isn't available on the HomePod (not yet, at least).
4. HomePod doesn't work well with other streaming services besides Apple Music
To be clear, streaming services like Spotify, Pandora, and Tidal will work on HomePod — they just won't work very well.
Once your HomePod is synced with your device, you'll be able to play whatever you'd like through the speaker, thanks to AirPlay. But you won't be able to control your music using Siri, which takes away a lot of the functionality that makes HomePod "smart."
5. HomePod can't hook up to another device using an auxiliary cord
HomePod does not have any inputs, so there's no way to plug in an auxiliary cord to listen on a non-supported device.
6. HomePod can't make calls on its own
Right now, if you say "Hey Siri, call mom," it can handle that in a matter of seconds. But in order to make a call using HomePod, you have to dial the person's number on your iPhone, then manually select that the call play through HomePod. It works great, but it takes a few steps to get there.
7. HomePod can't answer random questions
The HomePod version of Siri isn't prepared to answer random questions like Alexa and Google Assistant. It's not 100% clear what the device's limitations are exactly, but Apple says Siri on the HomePod is capable of "general knowledge." That likely means it can't answer obscure trivia questions, play games, or tell jokes like other smart speakers.