Never used the UC100 but most reports I have seen are favourable, there are many motion controllers out there from the very cheap and likely lower quality ones up to the higher end ones.
The advantages of an external controller is that the pulse (if it is a Step/Dir controller) will almost certainly be much nicer than you could ever hope to achieve from a parallel port, this often allows machines to run smoother than they previously did via the PP.
Another advantage is that the external controller does the pulsing so it frees up resources in your computer.
Also you can use a 64Bit OS, when using the PP the OS needs to be 32bit.
The disadvantage of the motion controller can be that it does not support some functions you require, ie the things that Mach normally does in the driver.
Things like Lathe Threading are not supported on all external controllers, THC, Probing etc. Most other "normal" functions of Mach should be fine.
Hood