TurnItIn does not give 'unreliable' reports - this is a silly categorisation based upon a misunderstanding of how it works and how we (staff) use it.
It is itself a bit of a blunt tool. It will give a high similarity for things which are, well, similar. In this sense it is eminently 'reliable'.
The issue is that 'similarity' does not mean 'plagiarised', yet most students seem to think it does. A high similarity just means that a submission uses a lot of word phrases that have been used in the same order elsewhere. This can be for numerous reasons: the similarities are references, thus book and/or article titles; they are commonly-cited block quotes; it is other assorted commonly-cited data; it is plagiarised content.
You only need to worry about the latter if you have a) incorrectly cited, or b) consciously plagiarised.
All a score in the 30s does os prompt a human marker to take a closer look at the details, which are accurately, reliably, helpfully highlighted and linked in the marker's report.