Compared to email, IMs tend to have less context and are generally short and to the point but otherwise unstructured. There's a pressure to respond quickly, and part of responding quickly is typing less words. Condensing thoughts to a minimum number of words is a harder skill than it seems; people tend to compensate for low quality communication with more words. Then there's still all the drawbacks of email: typing proficiency, spelling, grammar, and asynchrony.
Compared to mouth words conversations, IM doesn't have inflection or body language to help with tone. It's also not as fast as live conversations so it's easier losing context and easier to get overwhelmed with multiple conversations happening concurrently.
I personally love IM because it's easier to have multiple simultaneous conversations with more throughput and less latency than email normally achieves. I think of IM like async programming, when one conversation gets blocked, I can work on another. Mouth-to-mouth communication is like a singled threaded I/O, you're wasting resources (time) when you're blocked, but if you're not blocked you're more efficient since there's no context switching (so a heavily one sided conversation/lecture). Email is like multithreading, context switches are more expensive and the amount of work done per context switch needs to be big enough for it to be worthwhile. They all have merits and use cases.