Hi Kok Yong,
This is a very good question
However, it is very hard for me to answer this without going bit technical, so bear with me.
In Western Astrology, there are the 12 zodiac signs that everyone is familiar with, such as Taurus, Virgo, Capricon etc etc. In Chinese Astrology, we are more "kiasu". We subdivide the 12 Zodiac signs into what we call the 28 constellations. They are basically group of stars in the solar system.
This "Liew', (or rather "Liu" as the correct pinyin) you talking about refers to one of the constellations among the 28 constellations. Yes "Liu" is not a good constellation to get married in (by right...). But this 28 constellations system is no longer accurate.
For professional who knows date selection know that we no longer rely on this system for date selection. Reason being that (now this gets harder to explain without illustration.) the 28 constellations are actually degrees of latitude the Moon crosses during its monthly journey around Earth.Coz of the differences in the amt of time the moon passes through these constellations, some may be a few hrs, some may be few days.
Hence as you can see, in the tong shu, it is assigned one constellation to one day in the tong shu. This is incorrect. For eg, the Horn (or "Jiao") constellation is good for wedding. But it may only be good for only 6 hours within a day. After that, the moon will have already transited to another constellations on the very same day! If you assume you can use that for the whole day (which is as stated in tong shu), you will make a big big mistake if you choose a timing that is past the 6 hrs for your wedding! U will end up getting a bad day!
Hence this is one of the reason why the Tong Shu is no longer accurate. Even the bible of date selection, Xie Ji Bian Fang Shu, commissioned by Emperor Qianlong, dispute the validity of this system. It questions the basis on which the days are aligned with the constellations.
I know this is very complicated to understand, just as i find it very complicated to put it in words (haha), but i hope it gives you and everyone an idea one of the big reasons why the tong shu is NOT accurate