That's not relevant. For example, if someone has not taken a vow to abstain from killing, there is still a negative effect from killing regardless. The negative effect it has, is the reason why it's a precept to begin with.
Of course it is relevant. Of the five lay vows, only the vow against intoxication is prohibition rather than a natural non-virtue like killing, taking what is not given, lying and sexual misconduct. Likewise, the monastic vow against consuming garlic is a vow of prohibition, as are most of them, and so therefore, no non-virtue is accrued when they are engaged in by novices and lay people.
On the other hand, there is no vow forbidding malice, envy and ignorance, the three mental natural non-virtues because hinayāna precepts apply only to physical and verbal actions, nevertheless, indulging in these three results in negative karma and ripening. Likewise, harsh speech, gossip and calumny are natural non-virtues, but there is in fact no vows against them, because the Buddha judged it would be too hard not to break these and because there is natural negative consequences.
Most of the monastic vows therefore are vows of prohibition only and are not natural non-virtues. Thus there are no negative consequences if lay people do not follow precepts meant for the ordained. Otherwise, for example, sleeping with your wife would become a downfall.
Chan Master Sheng Yen just doesn't know what he is talking about? Sorry, I can't believe that. It's highly unlikely that he doesn't know what he is taking about. He's a well regarded monastic Chan master. He's also a scholar with a Ph.D in Buddhist literature. He knows what he's talking about when it comes to the sutras.
I am sorry, but there is no mention of negative mental effects of garlic in any authentic Mahāyāna sutra. There is a chapter of the Chinese Surangama Samadhi sūtra in the Tibetan canon, chapter ten, where it says that if one eats garlic, it increases desire, if one eats it raw, it increases hatred, but that is all.
I am not questioning Master Sheng Yen's knowledge of the Chinese canon. But I don't think there is any reason to accept what the Chinese Surangama Samadhi has to say at all. I don't consider it an authentic Mahāyāna sūtra. Just because a chapter of it is included in the Tibetan canon does not guarantee its authenticity. There are several texts included in the sutra and dharani section of the Tibetan canon whose authenticity is disputed. This is one of those texts.