I've been reading the cause of his abnormal lightsaber glow is due to a fractured crystal. Does that mean his lightsaber is less reliable in any sort of way? Was there any proof that his lightsaber brought him any stress during fights?
tl;dr: Kylo Ren's lightsaber is certainly less reliable. And while his weapon is unlikely to cause significantly more stress compared to being angry throughout a fight, he is likely not fighting in optimal condition.
While there is no direct proof of the above to be true, existing knowledge supports the above as a hypothesis to a large extent.
This requires some knowledge of lightsaber engineering. Lightsabers are feats of extremely precise engineering: beyond any organic to assemble by hand. It was only in the Imperial period when Palpatine was able to achieve a technological means of mass production - that's how long it took technology.
Up until then, only the use of the Force can allow one to perceive the crystal's matrix, then align it perfectly in the crystal housing to project a blade safe enough for use. Failing that will lead the lightsaber hilt to explode upon attempting ignition:
"Light this, and all that will be left of you and your lightsaber will be your crystal." - Prof. Huyang
The lightsaber is a handheld superweapon. You don't want it to explode in your hand.
Now, let's examine Kylo Ren's lightsaber's design. The crystal is fractured, and it cannot project all of the energy directed into it into the blade. What we once thought is a crossguard is known today to be exhaust vents - energy that didn't make it to the blade has to go somewhere. That "crossguard", basically, is what is keeping the lightsaber from exploding. What happens when something gets into the thermal exhaust port of a superweapon and disturb its innards that depend on extremely precise alignment? Remember what you don't want to happen in your hand?
You can now see why that is a technically unreliable weapon. I didn't even touch on how his lightsaber's energy is not in a closed feedback loop like modern lightsabers, because I'm not sure if that's canon anymore. If it is though, it also means his lightsaber has shorter battery life. More points deducted off the scorecard.
Impact on Wielder's Condition
The lightsaber crystal does affect its wielder's ability to achieve optimal conditioning for combat. The blade, when ignited, produces a certain resonance - the hum you hear. As noted in information on the Jedi Order's rites for The Gathering and subsequent lightsaber construction, the chosen crystal that a Jedi is guided to by the Force is the one whose resonance will be most attuned to him/her, thus allowing him or her to achieve the most optimal one-ness with the Force in combat. As for the Sith, they use the dark side to guide them in synthesizing their own crystals, so it stands to reason that the crystals a Sith made for himself would be equally in tune with his own Force too, since it's made by his Force.
Using a crystal not attuned to you, therefore, will mean you cannot achieve optimal condition in combat. While we cannot conclude beyond doubt whether the fractured crystal is "Kylo Ren's", there is reason to believe it is not.
Kyber crystals are extremely difficult to damage. They are impervious to physical damage (see above quote), even one of its own doing, and it can absorb any source of energy damage. The only recorded way of destroying one is through internal energy overload, destroying it from within.
The fractured crystal is unlikely to be attuned to Kylo Ren because:
- If it had been synthesized, it shouldn't come out fractured.
- If it had been found in a crystal cave, he shouldn't be led to a fractured crystal.
- He's unlikely to have damaged the crystal. (angsty emo teens aren't that powerful)
Note that "fractured" implies physical damage to an existing structure, not imperfections caused during formation (that's called warped). Something must have happened to the crystal after taking "normal" form.
Given the unique nature of the crystal, there has been speculation as to where it came from. While none are proven, they do suit well into the circumstances we know of in the movies, and therefore the probability of the crystal not being attuned to him is greater:
- The crystal may have belonged to Vader or Palpatine. Kylo Ren would LOVE to fight with Vader's crystal, the only component of the lightsaber that could have survived the Death Star's explosion. A limited edition piece of Vader memorabilia to "finish what you started" with! Palpatine's explosion and the Death Star's explosion combined may well be enough to fracture it.
- The crystal came from Snoke. The dude is ancient, the lightsaber is an ancient design, maybe the crystal came with the lightsaber. We know Palpatine supplied Vader with crystals as opposed to having him synthesise his own. It's possible Snoke did something similar (maybe to the extent of providing a fully assembled saber), though probably not due to any support for Kylo Ren's fanboy tendencies. In this case, the crystal's origins may very well be lost in ancient history.
In any case, not using a crystal he found or synthesized himself means he is unable to be fully one with the dark side, and so while not amounting to significantly more stress than being angry in a battle already is (when the Hulk already has an intent to kill, a buzzing bee in the ear won't change much), he certainly can do better (try programming in a crowd vs. in total silence).
- Wookieepedia articles on Professor Huyang, The Gathering, lightsaber crystals and Kylo Ren's lightsaber
- Star Wars: The Clone Wars (episodes on The Gathering and episodes from Season 6 that featured giant kyber crystals - that last one never made it to TV but is still canon)