1. WordPress embeds are not truely respnsive. Bad workarrounds like max-width: 100% on iframes let videos show in weird aspect ratios with black bars.
2. WordPress embeds are use oEmbed, and oEmbed **sucks** for videos embeds.
3. WordPress embeds use providers defaults, ARVE enhances embeds with usefull options by default.
4. WordPress embeds cant be aligned.
5. WordPress embeds give you no controll over them whatsoever.