The main reason I personally go for Et as a sub on my Fi for example, is the fact that I can skip [S4: Lifesteal Strike] and have a more damage oriented SAF like [S4: Frontal Adept] or [S4: Hammer of the Wise].
Other combination as Fi/Hu might do better burst damage, but Fi/Et gives me overall more consistent dps.
As for Tech classes, maybe going Et isn't a good choice, not because of the lack of healing but because of [Same Arts PP Save] and [Tech Arts Count Bonus] which are only for PAs.
But otherwise, as long as you are not a SMN or know your class enough not to get hit too much, it's just ok for tech focused ones, it's more like a niche on that situation.
My personal recommendation is to play with your main class enough to know the nooks and crannies of their mechanics, combos and tricks and try different subs and you will know which sub to go.
The thing about PSO2 is that the choices are really dependent on the player skill and playstyle, which I often compare it to fighting games...
Honestly, the rapid update in content and people on NA focusing too much on affixing made the majority of the player base to lack the experience on the class they try to play with.
That's probably the reason you can see similarly geared players on the same class and build. One trying to do ultimate Naverius solo and dying while the other one is hunting millions of points in endless rondo.
With time, experience and practice people tend to get a lot better, know the mechanics and have better reactions to the enemies...