A Rebbe of mine once spoke out very harshly against most mekubalim. He said that someone in his shul (this Rebbe is also a Rav) told him that he goes to a mekubal in Israel at least once a year, and everything he told him has come true. He said it doesn’t matter. The Torah is very clear that there are supernatural things which are not good, and are in fact forbidden. He contrasted it with R’ Chaim Kanievsky giving people segulos and advice on what to do: if you ask R’ Chaim what to do in an issue, he will give an answer and usually tell you what he based it on. For example, he told us a story of a young man who had gotten engaged. During the engagement, he was diagnosed with cancer. He wanted to break the engagement to spare the girl of having to go through the suffering with him, and she didn’t want to. They went to R’ Chaim, who told them to proceed with the marriage, and he said everything would be fine. He based it on the gemara where a Tanna (I don’t remember who) was visiting the emperor, and the emperor told him the case with a man who sold a field to someone else. The buyer discovered a treasure, and wanted to return it to the seller. Each argued that the other one should keep it.
The emperor said he would kill both men and tske the treasure for himself. The Tannah told him that he’d be punished for it – they should make a shidduch with their children, and given the treasure to them.
R’ Chaim said, we see from the gemara that if you want to do something to help someone else, at a loss for yourself, only good will come of it.
A mekubal might give the same advice, but it probably wouldn’t be based on the gemara.
The rebbe concluded by saying that this doesn’t apply to all mekubalim. There are many legitimate mekubalim out there who are huge talmidei chachamim, and there’s no problem with them. However, you need to do some research before going to see one. He also said that we shouldn’t need them – we have our own Rabbonim near us, and we should value them for what they are, and ask them our questions. Ignoring your own Rav to go to some unknown mekubal is an insult to your Rav.