Possible disadvantages
While the simplicity of ARC and audio-over-HDMI is great, it's not entirely perfect. The ARC standard was designed specifically to replace S/PDIF digital audio outputs, and thus supports every audio format that would normally pass through S/PDIF: Dolby Digital, DTS and PCM audio. It can handle both the TV's regular two-channel audio and 5.1 surround sound without any trouble.
But it also has the same limitations as the S/PDIF standard it replaces. Namely, it can't send HD or high-bit-rate audio used by standards like Dolby Atmos and DTS:X. This is especially irritating because downstream audio over HDMI can carry the signal with no problem; it's purely a limitation of the ARC spec.
Even more irritating, some TVs actually downgrade the audio output over ARC, converting everything to two-channel sound even if it originated as 5.1 audio. It's not common, but depending on your make and model of TV, you might actually wind up with lower sound quality over ARC. In these instances, connecting an extra audio cable or two may be worth the trouble.
What comes next: HDMI 2.1 with eARC
In 2018, HDMI 2.1 will begin showing up on TVs and sound equipment, and with it will come a new and improved audio technology, called Enhanced Audio Return Channel, or eARC. The biggest improvement eARC offers is support for full-resolution audio signal, meaning that it will support Dolby Atmos and similar uncompressed sound.
Unlike the original HDMI ARC, which works with all HDMI cables, eARC will require new cables that meet the 2.1 spec. And if you're looking to upgrade your TV early next year, be ready to shop for a new soundbar, too.
from: https://www.tomsguide.com/us/hdmi-arc-faq,news-26262.html