This is your typical less than $20 POS flare tool:
The biggest problem is that the clamp that holds the line is about 3/4ths an inch thick. But the ACTUAL CLAMPING SURFACE IS LESS THAN 1/2 OF AN INCH! The 45deg flare cone has to be bored out of that 3/4ths of an inch. So your tube slips back and you make an unacceptable flare, or you clamp down so tight that your crush the tube out of round. Again, if you spend an hour bending a line to fit, then screw it up because your flare slipped means you start over again.
This is a $295 tool:
Note the clamps in the lower left.. they are about 2 inches long and machined steel (as apposed to the $20 cast and poorly chromed clamp) and you can clearly see that the 45deg flare cone takes up less to 20% of the total length... that means a nice round, even clamping force the is about 1.5 inches along the tube... it is NOT moving around. The drawback being that you cannot have a bend very close to the end of the line. I'm sure there are professional tools with highly machined surfaces (or CNC devices) that can do it... but not at a DIY price point.
I'm not saying the $20 cannot make a good flare... but again, unless there is a real option that if you DO screw up an end and can cut it off/redo it, your nice just bent line is now to short.