When comparing IMO vs Hangouts, the Slant community recommends Hangouts for most people. In the question“What is the best team chat software?” Hangouts is ranked 13th while IMO is ranked 20th. The most important reason people chose Hangouts is:
If you have signed in your Google account, Hangouts allows making video calls with up to 10 people or one-on-one. There are business, education and government solutions that increase the participant count to for video calls to 15 as well as offers support for those accounts. Additionally, you can switch between using either the front-facing or rear-facing camera on your phone or tablet.
Ranked in these QuestionsQuestion Ranking
Pro Video call support
Pro Beautiful User Interface
Although IMO has a comparable, and in some cases superior, feature set when compared to the competition its key selling point is its interface.
From LifeHacker when it won "Best IM Client":
Signing on, navigating chats by tapping the tabs at the bottom, searching for buddies, and virtually any other operation is easy to find and use. The app, itself, is very quick. Even sending a voice IM, if you don't feel like typing, is responsive and sends quickly. Everything imo does it does well, but it's excellent navigation really makes it top notch.
IMO also has a fully functional website that you can use for chat while on your desktop or laptop. This really puts IMO ahead of the competition as you can use the same system on all your devices so you don't have to deal with overlapping notifications etc. (Note: IM+ also has this feature)
It should also be noted that it has the highest rating in the App Store due to both the UI and its stability.
Pro Easy to switch between conversations with tabs
Tabbed chats make it easy to switch between conversations
Pro Lists and chat histories are searchable
Both your buddy lists and chat histories are searchable.
Pro Accounts can be linked so you don't have to sign in to several accounts
Pro Push notifications
Pro Concurrent sessions
Concurrent sessions allow you to be logged in on your desktop while also being logged in on your phone.
Pro Chat groups
Pro Group video support
If you have signed in your Google account, Hangouts allows making video calls with up to 10 people or one-on-one. There are business, education and government solutions that increase the participant count to for video calls to 15 as well as offers support for those accounts.
Additionally, you can switch between using either the front-facing or rear-facing camera on your phone or tablet.
Pro Free and cross-platform
Google offers all features of Hangouts completely free on iOS, Android and web with quick and reliable sync across all platforms. It's also tightly integrated in other Google services - it can be accessed from services such as Gmail, Google+, Chrome.
While the standard Hangouts offering is completely free, there are business, education and government solutions that increase the participant count to for video calls to 15 and offers support that may cost money.
Pro No need to create a separate account if you use any other Google service
Hangouts uses Google account credentials. If you use any other Google service, then you already have an account set up.
Pro Chrome browser extension
Use the extension on any computer you can run Chrome browser to text from your computer or tablet!
Pro Excellent read receipts in group chat
It's possible to know what each and every participant of the group has read. Their icons are displayed below the last message.
Pro Save and sync your chats across multiple devices
Start chatting on your phone, then on your computer, then on your tablet seamlessly!
Pro Can share animated GIFs
If you have gifs in your mobile phone, you can share them in the conversation and your partner will be able to see it animated in the chat.
Pro Google Voice integration
Google Voice is a service that enables making and receiving free phone calls within US. Google Voice forwards incoming calls to all connected devices and allows placing calls from any of the connected devices, but they will show as coming from the Google Voice number.
Hangouts completely integrates Google Voice, allowing you to place calls and receive texts and voicemail from all of your devices. The Hangouts app for Android also includes a needed dialer for making calls with the users Google Voice number.
Pro Free calls to US and Canada from anywhere in the world
Pro Advanced status sharing
Hangouts can inform about the status of other users including what device they are currently using, whether they are in the middle of a call and when were they last seen. This allows making sure that you're not disturbing someone when initiating a conversation. It's possible to granularly turn off status sharing.
Pro Video tools
Hangouts can add different effects and overlays to video calls that can be viewed on your device. Unfortunately, they can't be set from your phone.
Pro Seamless integration with OK Google for voice control and typing
Hangouts can be used completely hands free, even if the app is not open and the phone is locked: "OK Google, send a Hangouts message to Mary".
Con No support for third-party instant messaging networks
On March 3, 2014, IMO will start discontinuing support for all third-party instant messaging networks.
Con Lack of end to end encryption
Communications are encrypted between client and Google servers but not from sending client to receiving client. This means that there is still a possibility for someone other than the receiver to read your message if they have access to internal Google servers. But it allows also lightweight chat history search, spam protection and other features.
Con Is owned by Google
Google is a cooperation that earns money from collecting data and breaching privacy.
Con Transitioning to corporate orientation
Google is turning Hangouts in to a Slack competitor. It's unclear what the future of Hangouts is, but Google has said the consumer Hangouts we now have 'isn't going anywhere.'
Con Poor interface
The app tends to lag, the design can be confusing and considering that another app has to be installed that supplies another icon in the home screen in order to get a dialer for calls in Hangouts, overall it is very confusing and cluttered.
Con No rich text editing
On Android you can receive messages with bold or italic text. However, you cannot bold or italicize text on Android, only on iOS devices or via the web interface.
Con No way to moderate groups
If a member of a group chat room becomes disruptive, or there's some other need, there's no way in Hangouts to remove somebody from an existing chatroom. Similarly, if somebody is no longer active, there's no way to remove that user from the chat room to make slot for new active members.
Con Buggy video calls
Whether video calls work on your device is pure lottery, and there's nothing you can do to fix it.
Con Poor indication of who's online
There is no way to find out if a person in your contact's list is online or not in the Android app.
Con Poorly manages data
Doesn't cache images well either received or sent. Often users will see the sent images re-downloading, same for conversations.
Con API support was removed
This change closed options for integrations and connections between similar apps.
Con Not available on Windows Phones
There is no Hangouts app for the Windows Phone.
Con Doesn't support sending voice notes or video recordings
Currently there is no built in function that supports creating and sending voice notes or video recordings.
Con No customer support
Free product, you're on your own.
Con Won't send SMS/MMS anymore
They removed support for sms/mms. The same goes with integration to hangouts.
Con No SMS/MMS backup
They don't have options to backup sms to cloud.
Con SMS messages sent from the web app can't be sent outside North America
SMS messages sent from the web app can only be sent to number in America and Canada - sending to foreign numbers won't work, although you can still receive texts from anywhere.