When my GF's father died unexpectedly she wanted to contact his soul to resolve unsaid matters, but she was unsure if that was a good idea to start with. The means 'how to' was not defined at that point. Back then we were in contact with a spokesman of the theosophical society so she asked him about the very idea.

His theosophical inspired view was like:

* there's a plethora of possibilities outside and inside yourself that could mimic being the soul of the diseased. Don't take for granted that what/who you encounter is legit, all while it seems like genuine. You can be misled.

* If you do happen to contact the genuine soul by plain luck, this could very well not be the "latest version" of the diseased, as older echoes of the diseased are all still in the library, you might have hit a random one of them. The message you obtain might be prone for an important update, an update you fail to access at that given time. Do not take for granted that you have contacted the final version of evolution of the diseased.

* Whatever you get as a message will firmly cling on you and it's near impossible to shed it. You don't want to glue a potentially questionable imprint on your very dear soul so: don't do it.

Imho his answers are applicable to our practices with deems or aya for the purpose of contacting the dead. Unless like Seeingisbelieving one incorporates firmly the smoke-and-mirrors possibility and put in perspective, in advance, whatever comes forth.