This is a point of contention with Space Wolves but the fact of the matter is that Space Wolf armies have to work to get the Ld9 standard of MEQ armies. This isn't to say the leadership of Space Wolves is bad, Ld8 across the board for basic units and 9/10 for 'specials' and HQs is still pretty good but that extra pip between Ld8 and Ld9 is a pretty large difference. That's an extra 11% failure rate (~72% Ld8 and ~83% Ld9) and whilst that may not seem much, over the course of the game that's quite a few extra fails - especially if you run lots of units. Now Marines generally aren't too fussed about failing leadership tests. With And They Shall No Know Fear they aren't getting run down in combat automatically and they can regroup under the most dire of circumstances and even gain movement when they regroup. However, failing those tests more regularly means your opponent gains more benefits from tank shocking, is more capable of escorting you off the board and means you are more likely to be pinned if your transport dies.
Again, not a major issue and lots of armies deal with Ld8 as standard such as Imperial Guard, Dark Eldar and Tau (hey...non-Marines, shock) but if you can easily get Ld9 for your main stay unit (Grey Hunters), you should. And this is of course where Wolf Guard come in and the associated 'cost' of Grey Hunters - not the minimum 18 point price tag for their babysitter but the use of an Elites slot which you may not always want to use. This is quite nearly mandatory to ensure your army is as reliable as other MEQ armies and means you have access to one less Elite unit. That's one less Iron Priest, Wolf Scouts, Dreadnought or Lone Wolf unit which plays a big role on how Space Wolf armies are compiled.
Now again, this isn't massive - so you don't get three units in your Elite slot, not the end of the world. You can still take a Wolf Guard unit + two of some other Elite and have redundancy. Hell, the Wolf Guard unit option itself isn't that bad and since you can detach members from the squad to other squads, you can still have a core of Wolf Guard as an individual unit left-over (i.e. 4x Wolf Guard w/combi-meltas + PFists for Grey Hunters, 2x CML Wolf Guard, 2x TDA Wolf Guard, 2x Wolf Guard w/stormbolters + WGBL w/Stealth Pants). You may have less flexibility compared to other Marine armies in using your Elites and maximising the potential of full Grey Hunter squads (not getting that 2nd melta if they are in a Drop Pod or Rhino) but five Grey Hunters + Wolf Guard still nets you two specials, two combat specials (potential three combat specials) and fits nice in a Rhino or Razorback.
Wolf Guard are also a great attachment to Wolf Scouts making them more reliable as a melta delivery unit and more fearsome in combat. The other common unit for Space Wolves, Long Fangs, doesn't need to worry about this issue being old and venerable they are old Ld9. However, there are other units which operate on this Ld8 principle. All of the 'Claw' units suffer from this and are only decent with a Wolf Guard attached regardless - still pretty meh though. The big elephant in the room though is Thunderwolf Cavalry based units including the unit itself, Fenrisian Wolves and Iron Priests (you take them on mounts right?). These units are relatively common (well not the baby Wolves though they are buffed by localised TWC regardless) and can therefore suffer quite a bit. Being smaller units mitigates this somewhat as well as not being inside a transport, you're not taking as many tests on these squads. That being said, their lower leadership can be problematic for a heavy hitting assault unit and is often why you'll find such units lead by ICs or running solo where such tests matter little. Again, this is not a major knock against them but something which needs to be considered during army development.
Space Wolves have a further way to alleviate potential leadership problems with Saga of Majesty. Both the major special characters (Njal and Logan) have this Saga and Wolf Lords + Venerable Dreadnoughts can take it for a paltry 15 points. Any unit within 6" of models with this Saga can re-roll leadership tests which helps a lot with both Ld9 and Ld8 units. I wouldn't recommend using this Saga as a replacement for Wolf Guard, they still add a lot of combat power to Grey Hunter units after all, but highlights whilst leadership can be an issue for the unprepared Space Wolves player, there are many options to handle this within the army book.
Again, just to be clear here - this is not a major issue of Space Wolves and generally something which is addressed during list development. It's really an annoyance during this stage and if not addressed, can prove drastically game changing on the table top. When addressed there is less flexibility in terms of list building at higher points where you use most of your Force Organisation Chart but for the most part your army is as reliable as any other MEQ out there.
