Skype for Business. Everyone I've spoken to in my company has had connection, audio, or screen-sharing issues. Personally, I consistantly have issues with what I've listed plus instances where Skype just flat out refuses to launch, or it crashes, or messages are randomly dropped or fail to send, or file/image transfers that just do not work. It is truly baffling.

I noticed another comment thread about Microsoft Teams, but for me, Teams is a godsend compared to Skype for Business.