I'm not going to get sucked into an internet argument but I'll respond to a few points generally and it will be my final response.
All right, then I'll make this my final response as well.
I'll give you this, although I have doubts on how you would have obtained this info. I've only used a demo of Gamebryo, so it's hard for me to be certain that it would require any major rewrites. However, poor performance on certain components is far from breaking the engine. The source code license is relatively cheap compared to MMORPG development costs, so I wouldn't say having to rewrite certain components would be a big reason to rule out the engine. It's far from ideal, but I'd definitely rather be able to rewrite certain components than being forced to make work-arounds without source code.
Certain engine core components being rewritten due to horrendous performance issues in various subsystems were done.
Have you ever played SWTOR? I can't say that the engine is at fault, but there are many reasons why SWTOR flopped after the first couple months of release, and why RIFT is still going relatively strong(especially if you compare the initial player base).
No it isn't an opinion in the least. The core renderer is nearly replaced in every game using it, the scene graph is cache miss heaven and the old way of doing things which doesn't scale with hardware, there is virtually no multithreading due to not having a job proper system. Also note that as I mentioned BigWorld and Hero Engine are proven solutions.. just because you don't like the gameplay or design decisions it rarely has to do with the engine.
I'd definitely prefer native control over the game executable in such a big project. Extensions are far from ideal for core components in most cases. I would argue you could not clone WoW or RIFT with either BigWorld engine or HeroEngine without source modifications or major hooks to the engine. It might be possible, but certainly wouldn't be easy to get that same level of performance.
You can also write extensions in C++ without a source license if you run into scripting performance issues. Also controls and gameplay in SWTOR are design decisions you could easily clone WoW or RIFT with it if you chose to do so.
Of course you can cross compile it to desktop, but the question is why are they marketing it as a web game. I haven't played any of these Unity MMOs, but I may give one a try soon to see what they have achieved in terms of performance.
You could just as easily cross-compile a native desktop application if you wanted to. Not to mention there is no 64 player limit unless you are going to use the default Unity networking subsystem... which they will even recommend you not to use and to go with Photon, Smartfox, etc.
Yes, it's a huge job to redo the networking. However, if you're looking to go big(which any MMORPG should be in my opinion), it's certainly worth considering the proven power of the UE core.
Then you've never looked at the source... the amount of work needed to modify the networking subsystem is extremely substantial. Ask anyone who has worked with UE3 how fun it would be decoupling the networking and building a new system. Sure it definitely has been done before but the amount of work is staggering. At the same time the OP isn't asking for a 6-7 figure engine... which you would need since you couldn't do it with UDK alone.
I would be curious as to how many modifications Bioware had to make with HeroEngine for SWTOR's development, but I'd wager a lot. A ready-to-go solution is nice, but can be very misleading.
Not at all, Big World and Hero Engine are the enterprise level solutions that come boxed and ready to go without extremely heavy modifications.
I'm envious of your experience. You may be right that using Unity would be a great solution for an MMORPG, but I'd just prefer a more controllable environment for such a large and expensive project.
Also for the record I've actually worked with GameBryo, UE3, Unity, Reality, and have worked on more than a few custom engines.
I never said anything about Heroes of Telara, but that could be relevant. Remember that heavily modifying an engine is still usually easier and cheaper than building your own.
I'd like to see some proof that RIFT doesn't use Gamebryo heavily.
Heroes of Telara runs on a heavily modified version of the Gamebryo engine - source
A heavily modified version of Gamebryo 3, indeed, with extra bells and whistles added on by us. - source
True, but the topic clearly asks what the best engine for an MMORPG is. I don't like going around to topics saying, "Well don't even bother asking this because you aren't anywhere near experienced enough to make <X>." Although that may also be an appropriate response, I prefer to answer the question without judging the person's motives for asking the question. He never asked if a beginner can make an MMORPG(like Mass Effect as he stated). If you tell him to use a free/cheap solution like Unity, he may put a lot of work into it in hopes that it will end up like Mass Effect, which would likely lead to disappointment.Not relevant -- we're advising a beginning developer who has little to no budget.
No development company with a large enough budget to make a decent MMORPG is going to use Unity for an MMO as anything other than a browser port.
My advice to OP: Use a free/cheap engine to learn only. Start with small projects, a MMORPG is a huge project in almost all cases. Once you're more familiar with the world of development, then decide what you want to do.