Well, the method worked for Juliana Tainer and it worked for Picard. Not to mention the previous transfers with Ira Graves in TNG and Drs. Korby & Brown in TOS which were a 100% success rate as well.
Distinctly different methods were used in all cases, so it makes good sense that there would be a range of advantages and disadvantages.
Tainer and Picard were the work of Soong, rather than of completely alien forces (pending more data on the Soong clan, of course). But Tainer appeared to merely be a very good imitation of the real deal, the best Soong could make, a bit like Daystrom made M-5 by abusing suitable pieces of a preexisting mind. I doubt it counts as anything more than a particularly well-designed holosimulation of a deceased loved one.
Graves was a bit of a surprise, even to himself: he clearly wasn't counting on a positronic template, this being the supposedly failed work of his rival. And the thing he was counting on, transfer to a more conventional type of computer, apparently failed him in the end: it's the reverse of Tainer, with the real deal becoming a dissatisfying reincarnation, rather than a fake being given the satisfying characteristics of the real deal. But the technique held promise, assuming there would be further Graves-Soong cooperation. This being complicated by at least one being dead and the other being a regular Scrooge when it came to sharing or publishing. A.I. Soong creating the working golem would be the logical conclusion of this nevertheless.
What the Camus II machine achieved probably wasn't reproducible with UFP science. And it was a bit of a failure in the end: it left both the victims confused and unable to logically further their respective causes, and soon got reversed anyway. Might be the user misset the alien controls; might be the machine never was intended to do anything more, and was just the Trek equivalent of the polyjuice potion. Certainly no transfer of mind to an atypical or artificial template was achieved.
Clearly an event occurred between the 25th and 32nd century that skewed the record in favour of not being a successful method.
Such as Soong dying/going into hiding and not leaving proper instructions? I mean, that's not just his M.O., it's basically his entire biography!
Timo Saloniemi