What are the pros/cons of the Dark Sigil and Hollowing?

Yeah, aside from the appearance, nothing really changes.

All rumors about losing souls if we die with a hollowing level of 99 (which is the max) are FALSE!

I just tried, having a hollowing level of 99, letting myself be killed, and I got all my souls back after retrieving the bloodstain.

So, no downsides apart from the appearance, which can be solved by buying an Untrue Dark Ring from Yuria at the Firelink Shrine after Yoel dies, which allows you to keep a human form even though you're hollow.

And if you want to heal completely, there are several tutorials on Google. But it requires you to pay for as many levels as you gained multiplied by the amount required to level up at your current level.

Hollowing affect weapons with the Hollow Infusion.

Weapons and shields infused with Hollow will boost the wielder's Luck based on the upgrade level and the wielder's hollowing. A wielder with 0 hollowing will receive no additional Luck, regardless of upgrade level.

Source: DS3 Wikidot

You have to pay the equal amount of souls that you would have payed for the "free" levels from yoel, meaning the max cost is the same as 5 levels.

No downsides to being hollowed, at least where stats are concerned. Indulging yoel just starts his quest-line and gives you access to more spells over time. If you want his full quest-line, make sure you DON'T heal the your hollowing at all, and check back with him for more levels every few deaths. Yoel's quest leads to some really cool stuff.