While research is inherently risky, there are two red flags:
He said that he thinks that idea is feasible but doesn't know what would be the approach, and not sure whether the results would be positive in terms of PhD because there several parallel domains.
What this tells me is that your advisor does not have the specialization to attack this particular idea that you are proposing; moreover, there are other areas of research that may be examining the problem that you are discussing.
There are a number of potential issues that come across from this:
You don't have an advisor who's familiar with the area, so you have to spend more time (either doing your own related work, or finding other faculty/collaborators) to discover the appropriate methods. Instead of having your advisor go, "Use this equipment and this research method", now you have to possibly discover what the method is and then acquire equipment for it. You will make mistakes numerous times because no one is there to help you avoid common mistakes. This makes your results take longer and uses more money.
If there are parallel domains, then there is a risk that the results that you and your advisor find from the work (that most likely took a long time because people are unfamiliar with the method) are possibly not as significant or as important as they first seem because this other area's already identified them first. Others familiar with the area might ask, "Why didn't you look at XYZ and ABC first?" and reject the paper. As a result, it will take additionally more time to familiarize yourself with these parallel domains in order to identify that what you're doing is important, relevant, and novel.
I think personally that (1) is a much greater risk than (2). Most people never come into an area with knowledge of what's there, so surveying related work, talking to people, and learning about the research area is usually work that has to be done anyway, but (1) can be very deadly. (1) and (2) in combination can be very dangerous, since it might indicate that your advisor simply doesn't have a lot of interest in Method (1) and in Result (2) - which means that your project might end up with less priority, less input, and less money. And all of that results in less papers and less impact.
Less papers, less impact means less opportunity to do important, meaningful work (and fewer job opportunities).
Thus, it does depend partly on what your personal goals are. If you do want to pursue this path for the sake of expanding your knowledge and the knowledge of the world at large, then one possibility would be to switch advisors or to find out who's doing this work in parallel domains and collaborate closely with one of these other researchers so you can get technical knowledge from those people.