I was looking at the herculien github where it is mentioned that this mechanism is superior to a corexy. Why is that so. I am intending to build a 600x600 x600 mm monster and i need to decide on the direction of mechanism to go. Any advice?
For that size printer I wouldn’t recommend going to a crossrod gantry of the Ultimaker or Herculien/Eustathios style. Corexy or perhaps Quadrap style will probably scale better at that size. Also @Jeff_DeMaagd has a printer that performed better than most small printers I have seen. And it has a massive build area: https://m.flickr.com/#/photos/jabella/25812195362/in/set-72157665553307120/
I didnt mean using rods, but to choose between the cross mechanism and the corexy. Both can done using plates or misumi sections and linear guides. Just want to know the disadvantages of corexy. Thanks.
My corexy was a bear to keep tuned. Balanced belt tensions was critical. And the long belts required for a 600x600 corexy might cause some belt stretch backlash or at minimum harmonic nodes along the long horizontal belt paths.
I was in a similar position as you a few months ago. I decided to go for a core xy for my 500x500 printer because of the scaling issue for a cross bar. It’s all about innovation. You need to solve those barriers. Once you overcome those corexy might be out performing a cross bar solution. We need more people to drive innovation and not just copying a design. Unfortunately not everyone is born with an engineering brain 
CoreXY is a very scaling-insensitive design. All you need to do aside from standard frame scaling is make the X bridge stiffer and use heavier-duty belts and bigger motors as the size scale increases. (Ignoring the Z stage design, which is basically identical to those used with cross gantries.)
Jetguy has a 1.2 meter cube CoreXY (four ballscrew Z stage) that makes incredible prints. I personally think it’s silly to lift the bed at that size, you should lift the entire gantry instead, but that’s a design trade-off.
Corex XY needs much longer belts for the XY , so - I wouldent go for any standart 6mm belts , you´ll need , steel enforced 8 to 10 mm belts , if not, you´ll see a lot of ghosting in your prints
If you’re looking to build, look up whether Gigabot has an open hardware version first, they’re a 600mm machine. A giant machine just isn’t easy do do from scratch, and scaling up a small design has many pitfalls too.
One last comment on the belts – CoreXY requires the same total belt length per usable size as a UM type cross gantry. It’s just in two long runs instead of four shorter runs. And when you factor in the 45 degree kinematic rotation effect on the idler pulleys, you get a sqrt(2):1 = 1.41x “gear reduction” between belt motion and gantry motion inherent to the gantry design. So [double belt length / sqrt(2) mechanical advantage] = sqrt(2) times or 41% more nozzle motion from belt stretch than an equivalent-size UM gantry. That difference disappears entirely if you use additional belts or cables to synchronize sides rather than UM-style torsion rods. Which is basically mandatory at large size scales, if you don’t want double the XY motors. Which means the CoreXY has minimal belt stretch disadvantage vs a cross gantry, if any.
For large printers, you may get better scaling with some kind of CNC structure. Have you checked the MPCNC? it has similarities with the UM cross Gantry, it’s quite cheap to build and has been used as a large scale 3D printer with sucess: http://www.thingiverse.com/make:172500
Check also this big comercial setup from 3DP, that looks like a CNC, and prints huge objects: https://www.youtube.com/watch?v=ZbWSUPgKgTI
i think the choice of gantry comes down to availability of belts… If 8mm belts and pulleys are hard to find then the only way is ulitmaker gantry with 6mm i guess. any body has links to good 8/10mm s2m/gt2/s3m belts and pulleys. the usual aliexpress stuff is horrible at this scale.
I have had great luck with everything I have purchased from Robotdigg. They have gt3: http://www.robotdigg.com/product/597/3GT+9mm+wide+open+ended+belt
SDP-SI sells actual licensed Gates GT2/GT3 belts. Fiberglass reinforcement so stretch is negligible. They have a 2.4m 3mm pitch 9mm wide GT2 loop belt that you can cut open for a ~500x500mm usable build area, for example.