Grumpy_Mike:
but if the LEDs have to be matched for brightness it won't work.
No so.
There will be very little difference in brightness with the same LED running at 20mA controlled by a resistor from 5V. Lower than a JND (just noticeable difference).
Any brightness variation will come from the LEDs themselves not the variation in current. As such constant current control will not help.
Indeed.
The eye can't detect small variations in brightness very well (10% for instance would be hard to notice) - it can cover a range of brightnesses of more than a million though - eye's response is close to logarithmic. The variation in LED die efficiency, lens position and so forth mean that getting accurately matched brightness cannot be done solely electronically anyway - and the efficiency degrades over time too!
The main advantage of constant current drive is when you don't have much spare voltage with which to program a current-limiting resistor (LED forward voltage depends on current and temperature and individual device variation, voltage across the current-limiting resistor is the difference between supply and forward voltage).
Also chips that do constant current mean you need fewer components (no resistors needed), freeing up space on the PCB - lowers the cost of mass-produced electronics.
Some chips combine constant current drive and boost or buck regulators - these provide the best efficiency for driving LEDs when the supply voltage and LED voltage aren't matched - but extra components are needed for the switching regulator.