I have to find a source for this.
I have been a Torah reader for about 45 years. My current shul has been in existence for about 80 years. I can vouch for the fact that of the 6 usable Sifrei Torah that we have (and we do, indeed, use all 6 at some point during the year,) I know that 5 of them were pieced together from either other Sifrei Torah or were written by multiple scribes. Considering their age, it would be almost impossible for such Sifrei Torah NOT to have been written by multiple scribes. I read from them, and, between checking with the shul's rav and the sofer, whom I speak to a few times per year, they are definitely kosher.
As a reader, while I know that such Sifrei Torah could not exist without this "piecing", I can say that it can become distracting when the writing style changes in the middle of a parsha that I am reading. I deal with it, though.
I don't recommend such Sifrei Torah for a "beginner" Bar Mitzvah boy unless he is comfortable and is extremely capable.