Soul tether needs some Chaos resistances and/or some other buffs.

It's a very nice concept for an item which could be a build enabler, BUT.

BUT.

Belts are an amazing slot for rare gear, since belts are the ONLY source outside of a couple of mana/flask nodes (near scion) which grant reduced flask charges usage.
Plus, belts normally grant a ton of resistances, attributes like strength, tons of energy shield for energy shield builds (crystal belt), tons of elemental damage (up to 42%), up to 90+ easy life points, flask life/mana recovery, even up to 5% movement speed, it's monstrous.

So, soul tether even REMOVES the natural starting ES pool and regen/recovery AND you lose 5% each SECOND. Dang, i guess it MUST have a preeeeeeeeeeeeeety good power for countermeasure right? Hm... meh... NAH, only average.
If only it was like 20-25% of maximum life as ES, that would be something for starters.
Plus, people with hybrid builds NEED chaos resistances a lot, which is hard to get, even more so without a good rare belt with it.

Plus, you can't eve use headhunter in a build based on it's leech ability. hahaha

Need more reasons?

I think someone else said it best. To paraphrase: This belt looks like it was designed to be balanced before the ES nerfs and was never adjusted afterwards.

(Disclaimer: I haven't actually used the belt nor even mathcrafted any paper viability for it, so I don't know if these criticisms are truly valid, though of course they seem so at first glance.)