For some reason, I've haven't really have a chance to see many Stela lanterns but from what I've seen, many after a couple of years have a couple of LEDs dead and some LEDs which almost give no output. What's the main reason for this? Are the LEDs all separate or is the panel (Aluminium PCB) that get's rusty in some places (or the LEDs are just rubbish so they just die)?

I would like to know the answer to this. If someone owns one, they will definitely know the answer to the "Are the LEDs separate or all together" question.

Stelas are early led lanterns (2008-2009), many lanterns from this era had similar reliability, the stela was just one of the first widespread led lanterns in the UK (meanin gthat it  may seem like more fail). I would bet that if you looked at 5 stelas and 5 of a different brand led lantern from the same era in a simmilar installation, their condition would be similar. Also I think that the stela has poorer quality, SINGLE diodes (this is just a guess though).

Always thought the Advanced LEDs Marlin was first as they date from 5th January 2010 

There are plenty of older leds than that, the leotek fx came from 2004, crees (betaled) LEDway ie from 2008 iirc, that mooncell enviro-lum is from 2003/2004. These are among many others.

We have about 10 Indal Stella fixtures which were installed in 2012. Back then they were not only the first LEDs in that village but also the first to have 3000K colour temperature. All of them still work flawlessly. Our troublesome lantern is the Luma range from around 2012 to 2014 which they were installed with bad Philips drivers from the Mexico factory. Many of these have been replaced in the last couple of years.
Luma lanterns from 2015 onwards are very good with almost 0 failures so far.