When comparing iMessage vs Element (formerly Riot), the Slant community recommends Element (formerly Riot) for most people. In the question“What are the best Skype alternatives?” Element (formerly Riot) is ranked 1st while iMessage is ranked 17th. The most important reason people chose Element (formerly Riot) is:
You're not confined within Element's or even Matrix garden, and you don't have to make users of other networks switch to Matrix.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Secure end-to-end encryption
Pro Cross-platform within Apple ecosystem
iMessage is available on iPhone, iPod, iPad, and Mac.
Pro Supports voice messages
Short voice recordings/messages can be quickly sent, and messages can be easily dictated into written messages as well.
Pro Free and unlimited messages over Wi-Fi and data
Messages clearly indicates whether messages are being sent over iMessage (blue) or SMS (green) where messaging rates apply. iMessages do not count as SMS messages with your carrier, instead they use a negligible amount of data to send over Wi-Fi and cellular data networks.
Pro Clear indicators for message delivery and receipt
The app indicates when messages have been sent, when they have been read, and when someone is typing.
Pro Comes pre-installed
No need to download or sign up for a new service.
Pro Seamlesss switching between SMS messages
Pro Friends don't need to "sign up"
You can just message them using their existing phone number or Apple ID and they will get your message.

Pro Clean UI
UI is minimalistic, and clear. And there is a plethora of stuff on the left of text box if you wanna use them.
Pro Apps and games for iMessage are available
For example, you can play games like pool or you can show someone a destination to go to.
Pro Bridges to other networks
You're not confined within Element's or even Matrix garden, and you don't have to make users of other networks switch to Matrix.
Pro Markdown support
Code snippets in chats can be highlighted with Markdown.
Pro Simple interface
Element has a very simple interface, adding the ability for more inexperienced users to use it.
Pro VOIP and Videoconferencing
Pro Supports encryption
Element allows for fully encrypted text, voice, and video chatting.
Pro Widgets support
Want to watch that flick at YouTube and discuss it at the same time? Have Grafana graphs stacked above your DevOps team chat? Collaboratively edit Google Docs and chat over without switching applications? This is possible with Element.
Pro Decentralized
An open network for secure, decentralized communication.
Pro Self-hosting and federated network
Pro Libre/open source
Pro Search
Search messages in your current room, or all the rooms you're in. Not subject to a message history limit like Slack.
Pro Cross platform
Web browser
Linux
OS X
Windows
Android
iOS
Pro Large existing community
With public rooms for many people, and you can create your own and let people from the community join.
Pro Etherpad real-time document collaboration
An easy to activate integration that allows multiple authors to edit a document simultaneously.
Pro File Sharing
Pro Supports text, voice, video
Cons

Con Not cross-platform
iMessage is not available on anything but Apple devices.
Con Can't send separate texts to individuals who share a Contact record
iMessage is an utter disaster. It can't send separate texts to anyone who is on a shared Contact record but with separate mobile phone numbers (e.g., "Paul and Angie Smith"). It merges ALL threads that ever went to ANYONE on that shared Contact record. Want to text just the husband to set up a surprise party for the wife? Not possible. iMessage will ONLY add that new text to the EXISTING thread you had with the wife. Duh! iMessage is pitifully incompetent.
Con Blocking Spam Messages takes 3 steps more than is needed
You have to open the message, select the sender, open info on the sender, then block sender. The default Android message app, select the message, block sender. it automatically deletes the message and blocks the sender.
Con iCloud sync of message is not stable
Sometimes, the messages on different devices with the same ID will be different, the syncing function work poorly.
Con Slow and unreliable
