The best post I could find, aside from the now deleted topics about how terrible HeroBlade's audio problems are:

Lord_Kreegan- "From personal experience, it takes a very long time to build an engine from scratch -- which is why game companies don't and other companies dedicated to the business of making engines do.

There is an enormous number of similarities between the "stock" Hero Engine and ESO's engine -- similarities most readily apparent in terms of the engines' shortcomings -- so I have no doubt whatsoever that ESO is using major "parts" of the engine. It can be purchased in that fashion: where you get the source code and can do with it whatever you want. EA/BioWare essentially did that for SW-TOR, although they went with an early release of the code.

Notice that both games are zoned, have problems (network lag) with multiple players in the same local area, suffer graphical hitching in complex scenes, have very limited and linear game play in terms of types of quests, have memory management issues causing slowdowm after playing for extended periods of time, are event-driven to an extreme, suffer loading screens (that can be "inifinite") in the middle of nowhere for no apparent reason... I could go on, but it's not worth it. The white knights will simply declare the problems are common to all MMOs; the fanbois will declare they never have any of those problems; etc. ...and admittedly, the similarities are not conclusive evidence of related engine use/development.

Primarily, the Hero Engine is "middleware"; that is, you build your own code around it. In ZOS' own words:
while we were prototyping the game on HeroEngine, we were simultaneously developing our own client, server, and messaging layer that were specifically designed with ESO in mind
... Considering the usual marketing doublespeak we get from ZOS, that statement basically tells me that they built the "wrappers" around the Hero Engine code. There's a whole lot more than just client, server, and messaging in an MMO engine.

And, noting that ZOS was being rushed at the end there before release (which is why they released the game in such a sorry state) because they were running out of money and the investors were getting anxious, it's easy to surmise that they went with what they had "as a whiteboard".

The Hero Engine is not a bad engine; it's just not a great engine. EA/BioWare has spent a lot of time and money "fixing" its game implementation on top of the engine. ZOS hasn't had enough time to fix everything."