There's a difference between studying law and practising law. I think some people advise against both, some advise against only one of the two (in some jurisdictions you can practise without spending your uni years doing a law degree), some advise against practising for too long. So it all depends on who you speak to and what their background is.

My guess is that a lot of people that become lawyers generally could have pursued a wide variety of other studies at university and also probably studied quite hard both pre-u and during university (to the neglect of other activities such as socialising/having fun and developing knowledge/skills in other domains).

So I suspect there's sometimes this regret of spending all that effort and then finding themselves (in their own perception) stuck in a career they don't like when they could have done so many other things? This will be be the case especially when kids (many of whom scored top marks at pre-u level) get grinded down to below average of the class in uni or starting their career with firms that don't pay or train them as well as they think they deserve (or both). That is perhaps one explanation apart from the usual stories of long hours and pay that is incommensurate to the effort etc

Also maybe lots of people complain about their job/degree but perhaps due to law having some status people pay more attention to it?

Having been doing this for a while, another possible issue with some Singaporean kids is that they might not have thought hard enough about what they want to do in their lives and why. Some of these kids choose a degree / vocation based on some graduate salary survey or the perceived prestige/popularity of a degree. e.g. CS is the hot thing now. It's not necessarily going to lead to satisfaction for everyone.

Having done both law and non-law I don't regret any of it.

And to your last point: i think in this forum most people like to compare to those that make more money than them, e.g. investment bankers, techies? but then i wonder why be a professional (subject to all sorts of codes and ethical rules) if the aim is to mint money?