Is there a difference in difficulty in singing specific vowels? Yes, and we need to talk about why this happens. Start by clicking on the link for a chart of vowels that has sound samples of each vowel.
In the chart that the link above brings up, click on the outside-right samples on the chart. From bottom to top you have the vowels: ah, aw, oh (flow), oh (yawn in standard British English), and oo. We call them lip vowels, because the sound changes mainly by changing the shape of the lips.
Now, click on the outside-left samples on the chart. From bottom to top you have the vowels: a (bath in New England English), a (bat), eh, ih, ay, and ee. We call them tongue vowels, because the sound changes mainly by changing the height of the tongue.
On both sides of the chart the vowels ascend from open to close. Try singing the vowel oo. You’ll probably find it more difficult to get a nice open tone than for ah. Likewise, you’ll probably find a easier to sing than ee. Then, we have the neutral vowels.
In the middle of the chart you’ll see a letter that looks like an upside-down e. Click on the sample to hear the schwa vowel (uh). The schwa, being a neutral vowel, can be difficult to produce with a strong tone. Likewise, the R vowel (earth) is so difficult that my singing teachers suggested avoiding singing it with a sustained tone.