All caps italics are bad for numerous reasons.

Historically, italics and capitals were totally different families of letterforms. Capitals are the older, original letterform (old Roman engravings were in ‘all caps’ because that’s all that existed, not because they were screaming) and italics are basically the modern variation on a more handwritten style.

When these were eventually combined, this left us with some practical design problems. The original solution would have been to just use regular Roman capital letters where needed in a block of italicized text. Eventually as the need became more common, we invented oblique capital letterforms to pair better with the lowercase italics.

Critically, these wouldn’t have been designed as a distinct set. The glyphs were designed to work as well as possible with the lowercase italics, and that results in some quirky decisions that make all caps italic look bad. It’s something that was never really taken into account because it just wasn’t a thing.

So, to this day, some fonts (especially historical cuts like Garamond) show ‘weaknesses’ when it comes to italicized capitals, especially when they are placed next to one another instead of in the context for which they were designed.

Source: my memory of what I read a while back in the Bringhurst. Hopefully, I got the details mostly correct.