Practically everything else.
As an IM client, it's abysmal. I have the mobile app on my iPhone 7, but the conversation histories on my desktop client and my iOS app don't sync. Even the real-time conversation doesn't sync. This means I can have a conversation with the same person on both devices AT THE SAME TIME and they will have partially sync'ed message histories, but never fully synced. How is it possible this can happen? Thus, the only way to retrieve the full conversation history is to browse to the Conversations folder IN OUTLOOK, where it's stored the moment you go idle for more than 2 minutes in a session.
You can't send files through Skype for Business. Don't even think about sending images either. This makes collaboration a complete inconvenience, because you have to rely on Outlook to send media or documents. And if you work in an industry that is very particular about IT security, like banking, then I feel sorry for you because you can't even rely on other, better, file sharing and collaboration platforms like Slack, Box, or Dropbox.
The application crashes at least once a week, usually more.
The web conferencing features are similarly abysmal. There are a few major problems with its web conferencing capabilities, I'll go through the ones I can remember here.
A) Are you meeting someone from outside your organization? Good luck. No one uses Skype for Business in the rest of the world. What will happen is that you'll spend 15 minutes at the beginning of your conference call figuring out that your external party can't dial in, and everyone will get all stressed and frustrated in the process of quickly jumping onto another conferencing platform, and you will be late, and your meeting time will be reduced, and the outcome is just negative all around.
Actually, external users can dial in to your Skype for Business meeting... IF your company's IT team has enabled this feature. https://support.office.com/en-us/article/Add-an-external-contact-in-Skype-for-Business-b42d26d0-82d5-4aa6-901f-aeab00b15650
And if they also have Skype for Business, they might be on the wrong version, or their IT team might have configured the federation policy differently, and you'll still spend 15 minutes figuring out that you can't both dial in to the same service. https://www.linkedin.com/pulse/allowing-external-users-join-your-skype-business-meetings-saad-/
B) No webinar features. Are you trying to brief your management or team leads on the latest X or Y, or have that quarterly town hall? Guess what, Skype for Business has no dedicated webinar functionality. This means that:
i) You have no attendance analytics.
ii) You have to MANUALLY mute all audience members, and you have to remember to do this otherwise that one person playing music in the background or using that extremely loud keyboard will disrupt the entire webinar. And anyway, they can just unmute themselves.
iii) No organized way of handling questions, or "hand raising" functionality.
C) I already said this, but it isn't stable. It regularly crashes, or the presenter will suddenly mute.