It is probably the Czech people who sold their vowels! I totally agree with you on that Jeremy! I notice the abundance of consonants in the Czech language myself, too!
As for the Hungarians, they have never been Slavs
Although they borrowed some Slavic words, mostly in names of some fruit or vegetables. What makes me smile, they also borrowed the names of Wednesday, Thursday and Friday from Polish
It is "szerda" (pol. środa, the middle day), "csutortok" (pol. czwartek, the fourth day) or "pintek" (pol. piątek, the fifth day). If you ask a Hungarian about the etymology of their Wed - Fri, they simply do not know!
Polish is known of having several rustling consonants: sz cz dz rz ż. Each of these letter combinations is a single sound. We also have softened consonants such as ć or ś or ź. That makes the language hard to learn by foreigners. Take "szczęście" (happiness). Sh-tsh-en-... and I even do not know what to do with "ś" to transcribe it to English!
Still, there are Czech sounds I am unable to utter. The word "a cross" is "kříž" in Czech, and it is "krzyż" in Polish. If I try pronouncing it the Czech way, they laugh at me and say I will never make it!