Wear and tear effects your hull, weakening it by a maximum of 30%. You don't visually see it though, so even though it may say you have 100% integrity, in reality it will be 30% weaker than normal (once you reach that threshold).
What I have noticed is that once you have reached that 30% weakening, every time you take heat damage or exit supercruise too fast, your subsystems seem to take more damage.. instead of the normal 1-2% damage they take, a ship fatigued by wear and tear will suffer 3-5% damage on each subsystem when suffering a fast supercruise exit or heat damage. I'm not 100% sure if that's related to the weaker hull though, not seen it mentioned by a dev, I'm just going off what I experienced on a couple of long range trips I took, a 29,000LY trip in gamma before wear and tear was implemented and a 31,000 LY trip after release when wear and tear was active.
All it really boils down to is how competent you are at fuel scooping, avoiding heat damage, and not exiting supercruise at too fast a speed. All are easy to do on short or medium trips. But if you're making journeys that take 1,000+ jumps then its more likely that a handful will catch you unawares - especially when jumping into white dwarf systems as they can quickly cook you if you're not paying attention.
Wear and tear makes the journey home much more difficult now, as you will be at your most vulnerable with your ship fatigued and weakened. So the most dangerous part of any journey is the last few hundred light years when your're heading back toward the frontier. If your thrusters are degraded and drop below 80% integrity, its much harder to win interdiction attempts or boost away from a hostile player/npc. Field maintenance units are useful if you plan on being away from repair facilities for some considerable time. But even they won't repair hull or thrusters.