The problem with JSE, is as Yoshi-P discussed some time ago. Is that if it's powerful enough, once acquired is it permanent BiS or mandatory gearing. Which removes it's uniqueness. Or if it's too weak, it'll fall out of favor and the development time spent designing and balancing will have been wasted.

Flat bonus' is either to static to have an issue as you level or too large and skewes lower gear tiers.
-A increase or added effect to an ability will be replaced as gear gets better and the effect is negated by greater stats.
Scaling bonus' is either to powerful and that piece becomes permanent and mandatory until a massive gear reset.
-Once you look past the neat text, all a Scaling bonus is just +Stat to specific ability or abilities.

This type of gearing would prove problematic during relevant content. It's power and unbalancing design would negatively effect the meta of the game.

The closest way to balance this type of gear is to use the WoW approach. Create the bonus' as a set type gear and limit it's use to either added effect, or increase in current effect. Requiring multiple pieces of lower tier gear to be equipped means that players can't choose to use an overpowered bonus in higher gear content, since your overall stats will be lowered vs. an appropriate tier geared player.