'Feudalism' is a term which should be used very cautiously; it's extremely doubtful whether 'the feudal system' was ever anything more than an academic construct. There's a good WH thread about this somewhere; I think LightSpectra wrote it.
Feudalism was of course not a homogenous system which existed all around Europe in exactly the same shape and form. Feudalism is a term used to denote common or very similar features of organization of various Medieval societies all around Europe (and also post-Medieval societies since in many countries some elements of Feudalism survived even up to 19th century).
The same case was with slavery - as you already wrote. Slavery in one civilization was not equal to slavery in other civilization, etc. But all of them had that common feature which was owning slaves and having their economy usually to significant extent dependent from / based on slave labour.
Western European historiography has developed a much different definition of feudalism than Eastern European historiography - I stick to the latter because it is much more flexible, more fitting to historical reality of the period, and allows us to describe more various Medieval societies as "feudal".
Recently Western European historiography has indeed started to use this term very cautiously and started to have doubts whether "the feudal system" - as they define it - was ever anything more than an academic construct.
Indeed, Feudalism as defined by Western scholars has never been anything more than an academic construct because their definition and understanding of it has been too narrow, not fitting to the historical reality of huge regional differences in social organization in Medieval Europe, and simply erroneous.
Especially the Western academic concept that Feudalism was everywhere the same in Western Europe, maybe except of England where it was slightly different than "in the Continent". Regarding Feudalism in Eastern Europe - Western scholars were never really interested in researching this subject.
for the most part, Roman slavery was an awful lot kinder than Greek, for example.
In what way exactly?