I learned years ago that the best way to check tension is to do a zigzag. That will show up problems with tension that might not show up in a straight stitch.
My experience has been that almost all tension problems are due to people not knowing how to adjust tension properly on a machine. Over the years I bought quite a few thrift store machines and fixed them up for donation to a non-profit. I was always able to adjust tension sufficiently well on those machines for people to use them for piecing quilts.
The only non-user issue I can think of regarding tension is if a machine has worn out parts or was so junkily made (with very poor tolerances on parts) that it cannot maintain tension once it is set. The former can typically be fixed by a technician; the latter is simply a characteristic of really poor quality machines.
One thing to note about modern machines is that it is possible to damage the bobbin area enough to require replacement. This happens especially when people use metal bobbins in machines that came with plastic bobbins. The bobbin areas of older machines are all-metal, whereas some of the components in more modern machines are made of hard plastic. The hard plastic is fine for plastic bobbins, but metal bobbins can damage the plastic.