Foreign learners do not generally find Russian numbers to be "straightforward". Not only do nouns qualified by numbers take different numerative cases (the genitive singular for 2–4, the genitive plural for 5–10), but the declension of compound numerals is also surprising for speakers of many languages.
Compound hundreds are also unpredictable. Yes, historically the varying forms are clear (the back yer in Common Slavonic sŭto ‘100’ was lost or strengthened depending on position), but learners today without any background in the Slavic languages simply need to learn them by rote.
True, sound changes like dvesti÷trista÷semsot make learning harder, but i'd say that's not really the property of the number system but of a language as a whole, since similar shifts are present in other places too.
For me Russian numbers were simpler than French because you could learn the basic idea quickly, and then pick up subtle sound shifts by watching tv/reading.
(disclaimer, i am not a native speaker but learned Russian at school age so may be misgudging the difficulty of the language)
Compound hundreds are also unpredictable. Yes, historically the varying forms are clear (the back yer in Common Slavonic sŭto ‘100’ was lost or strengthened depending on position), but learners today without any background in the Slavic languages simply need to learn them by rote.