Using VoiceOver on iOS, I primarily use Messenger and Messages for chatting. Apple's Messages app works perfectly fine, if a tiny bit cluttered with new messaging modes near the text field, and Facebook Messenger is also decent, but much more cluttered and filled with many more inaccessible elements, like the gif keyboards, stickers, and all the crap they have piled around the chat text field interface. WhatsApp works fine and as expected with no major issues for me. Google Hangouts on iOS used to have terrible accessibility issues, but they seem to have since ironed them out for the most part, but I haven't used it enough to give more informed feedback since the majority of my messaging happens through text or FB. Slack, on the other hand, is an absolute mess. Ridiculously difficult to get around the interface, the chat window has a habit of jumping a month backwards in messages if you so much as look at it funny, too many hoops to jump through in the UX to do simple things, unlabeled or poorly labeled elements, and they claim to have accessibility at heart but have made no strides to fix the most glaring issues in the app.

On my Mbp using VoiceOver, the Messages app is accessible, albeit requiring a little more interaction to easily get to and from different chats. I hate using Google Hangouts and Facebook Messenger on the computer as I find them to not be laid out in a logical way that easily serves a screen reader. Too many hoops to jump through and interact with to quickly and efficiently navigate to and through conversations. I much prefer it all on my phone. Slack on the computer is an absolute no go with a screen reader. It is way too sluggish and buggy to be worth any time at all, so I always defer to the iOS version when doing any IM for work.

Edit: When it comes to web chat interfaces, I do my best to avoid them. Recently had the annoying experience of trying to interact with the Microsoft Store help bot interface, and it kept tossing my VO cursor to the top of the chat history, making it difficult to figure out where the actual responses were, and made it all but impossible to find or figure out that interactive buttons were appearing to advance the conversation. Any web chat interface must use aria live regions in order to properly communicate text and interface updates to the screen reader without compromising where the user has the cursor located.