It is permitted to pronounce the sounds a little differently, for example v as ʋ or s as z, whilst the sounds from the table above are the default.
The runic writing is not meant to be used in regular texts. Use it only for purposes like art, etc.!
We have discussed many times whether to include a x-like sound. On one hand, without it, some words were not easy to derive (e.g. the Proto Germanic nahts (English: night)). On the other hand we found that the sound is very disconcerting to those, who don't have it the phonology of their mother tongue. Eventually we have decided to exclude it from the draft, since it seemed to be too much of a trouble maker; finding words was not that much of an issue in the end.
The letters ö and ü were not included inconsiderately. Factually every Germanic language has an ø-like sound in its phonology, but the main reason for the inclusion of these sounds was that altering the vowels of affected words to "near" vowels (e.g. i ← y → u) most of the times resulted in disconcerting and unsatisfying creations. However the modern Germanic languages often feature vowel shifts throughout their natural evolution; some switching to the one and some switching to the other. Therefore we felt is was more convenient to have these sounds as compromises and connectors between the languages.
We also struggled to find proper written representations fo these sounds. One possibility was to use the IPA-like symbols ø and y, like e.g. Norwegian does. Unfortunatelly this had taken away the possibility to have the alternative writings oy and uy, which we considered important, since not everybody can easily type non-standard Latin letters. So we arranged to use diaeresis, like German and Swedish does.