They are similar to other German/Euro cars...expensive parts, somewhat needy, intolerant of poor maintenance.
I've got a well-cared-for example of an '03 MCS with 92k and a Dinan S2 package.
Fun car.
Watch out for mushroomed strut towers...the surfaces should be perfectly flat and the strut mount bolts should be parallel, not pointing away from each other. Solution is aftermarket strut mounts, strut tower reinforcement plates, and some quality time with a BFH and block of wood to knock the strut towers back flat.
Later Gen1 cars had leaky passenger-side hydraulic motor mounts.
Electric power steering pump failures.
Crank pulley / dampener failures on early cars.
Crankshaft pos sensor o-ring oil leak.
HID failures on early cars. The ignitors are integrated into the $900 headlamp buckets and aren't replaceable separately.
Water pump is gear-driven off the back of the supercharger. Gear is sealed into the SC housing. SC oil isn't serviceable without removing/disassembling the SC. Some have had the couple of tablespoons of oil in this gear housing seep out and the gear grenades. =no waterpump and SC rebuild.
Clutches last about 80-100k. The flywheel is a sprung, dual-mass design. Feels great, but the clutch/flywheels is expensive and tedious to replace.
The Midlands 5spd in the non-S and the CVT have had some bad reports.
That said...the engines and transmissions (Getrag 6spd is the same unit as the Focus SVT I think,) seem to be solid.
I haven't seen many reports of engine or HG failures. Go over to NASIOC and you're read all kinds of doom and gloom stories of spun bearings and other engine failures.
Did I mention how much FUN it is to drive? I get about 32mpg mixed, with 205hp.
It isn't a Toyota, but I think lots of people treat them like one and flock to the internet to complain. If you go into it with the expectation of a Volvo-VW-Audi-BMW ownership/maintenance experience, you'll be delighted with it.