My understanding is that model 620 and lower Touch & Sews have metal gears. New plastic gear sets are available for machines equipped with them. I like the 2 Touch & Sews I have. A 620 and a 648. The 648 needs new gears. Plastic gears in any sewing machine can be a problem. Gears aging and becoming brittle isn't a problem that only Touch & Sews have.
The 620 uses both a built-in camstack and the separate cams. They can be used together to create even more stitch patterns. The 648 is much simpler. Touch & Sews are very capable machines when set up correctly. To me it's a requirement that you read the owner's manual before using one though. They're not as intuitive as other machines to use.
Rodney