It's a very classic Marshall from 70-80 era. The tone knobs don't do much, and the amp has basically one basic sound and one channel. At least Marshall added effects loop, but for 800 euro amp it's not exactly packing a lot of features.
But oh man what a sound it is, and the reason why people want these. Literally pure rock'n roll. It makes you want to play AC/DC riffs all day long. Very "AC/DC Back in Black" type of vibe, or the band "The Cult" from 80s. No one buys these amps to get a nice pristine clean sound.
What people also probably like is the fact SC20H isn't a high gain amplifier in any sense of the word and it doesn't have a super saturated or compressed sound. It's the middle ground between the "old school" and high gain amps. Throw in an overdrive pedal, and you get to the Slayer and 80s Metallica domain. Even then the sound and separate notes retain very nice clarity.
The amp is also very loud even in 5W mode unless you use an attenuator, and doesn't really sound good at lower volumes.
It is a great amp, but to be really honest it's worth 800 euros only if you really, really want this sound and don't want to spend lot of time tweaking other amps to get the same. You can get a DSL20HR sound similar, for example. It has a harsher and bit more modern sound and also way more gain.