For example, if your lines on the matrix side are long it converts them into urls. So people on IRC end up seeing you talking with 1/4 of your messages just being urls to some random server's pastebin.

When the matrix gateways go up and down you'll have 1000 random users thunderously join and part the channel at once.

Matrix comment edits flood the channels.

Matrix is unhelpful with abuse reports, in my experience.

I generally ban matrix from IRC channels for these and other such nuisances.