Underworld is totally different to Sanctus.. Sanctus is very very straight out neoclassical..Underworld is very very proggy, heavy, dark, fantastic!!
As for SyX clones..absolutely not!!!! Stephan does a couple of similar riffs every now and then to Romeo, but its re-arranged in his own style and unless youre a SyX fanatic and know every little riff Romeo's ever done, dont think you'd even notice it. But as for style and sound, its completely different.