With the two plosives, /pb/, in the middle, it is quite hard for English-speakers to pronounce and you end up with it sounding like two separate words and you almost end up with each syllable getting its full value /kʌp.bɔːrd/.
With it split as shown, "cu-board", it is difficult to pronounce without putting the stress on the second syllable, /kə'bɔːrd/, as we tend to make unaccented syllables neutral. Of course there is a spectrum between a full vowel and a neutral one so it may not be exactly a neutral vowel.
But I would say "cub-oard", with stress on "cub" and thus the second syllable would become fairly neutral, /'kʌbərd/. You cannot put the stress on the first syllable in "cu-board" as we cannot say /kʌ/ by itself in English. (I would say /'kʌbərd/ but that is my dialect!)