I like to think that my contribution to the community comes in the form of writing clear (if a bit long) articles that are beginner to intermediate in their levels as far as gaming goes. Every now and then there's a hobby tip, but I'd refer you to better places (like the Painting Corps, for example, or most of the main articles on From The Warp). I think there's nothing wrong with admitting your weaker areas, but moving along.
I think people have started to notice it on some level or another, but a lot of the older 'dexes (IE: Dark Eldar, Necrons, Tau, Witch/Daemonhunters) have a lower selection of effective units than newer ones (IE: Space Marines, Imperial Guard, Eldar to an extent). Why is this?
Older Rule Set: Dating Yourself
It's like any of your older co-workers dropping a reference to a show/play/book/song you didn't know existed: some stuff is just old. It was designed for a different gaming environment, in some cases. For example, look through the Daemonhunters 'dex and note Grey Knights have an ability to deal with minor psychic powers. Anyone else wondering what a minor psychic power is? I am. Same goes for the 'nid codex Carnifex Tail Upgrades, that give you attacks based on being outnumbered.
Some stuff's just plain out of date.
Older Tactical Game
The rules have also changed the nature of the game in terms of tactics/strategy. Mechanization and True Line of Sight (TLOS) are major changes from 4th to 5th edition. Players with a longer, more illustrious history than I can probably tell you about 3rd to 4th changes, though the number of codices written from that time are fewer. (I think Dark Eldar and Necrons qualify, though, and probably the Inquisition 'dexes).
These changes are not always negative, though. Consider the Dark Eldar; Nightshields on Skimmers and the ability to bring an absurd amount of Dark Lances makes them an effective (if highly, HIGHLY inforgiving) army. Even the vehicle rules help out their AV10 vehicles, and their amazingly well-armored (read: AV11 front) heavy support hover 'tank'. The Tau, too, with 72" Rail Guns + TLOS made out well, since they can pop you if they can see you on a 6x4 board.
Tyranids, though, have some problems killing vehicles. Since glancing is no longer an effective way, you're looking at a scant handful of ways to kill vehicles in the whole codex:
-Zoanthrope Warp Blast (18", and can be countered as it's a psychic power)
-Carnifex S8 Barbed Strangler (S8, and either BS2 or BS3 blast weapon...)
-Warrior Deathspitter (yay, S6 small blast. Not really that useful unless it's a Rhino or less)
-Carnifex melee attack (S9 + 2d6, but it's on a slooow platform)
-Genestealer Melee (S4 + Rending on Fleet; that's about as good as it gets)
-Warrior w/ Rending Claws (...still iffy; expensive platform.)
Orks? Orks pretty much HAVE to punch it out, and that means Power Klaws against vehicles. Lootas are passable at lighter tanks, but larger stuff? You're stuck with largely innaccurate shooting.
Just Plain Bad Writing
I touched on this in my Eldar Hall of Shame article, though you get some of the same in Mercer's Necron Tactica: some stuff just isn't that great. Older codices are chock full of units that just are not that effective.
For example, while the Tau are effective in some builds, there are a chunk of units you should simply forget exist:
-Ethereals (including Aun'va)
-Gun Drone Teams
-Kroot Shapers/Krootox options
And, some of their other units are questionable for effectiveness at best (IE: Stealth Suits, Sky Rays).
Bottom line? There's a difference between a well-written codex and an effective army.
The Newer Guys
On the other hand, some of the newer codices are coming out chock full of decent units.
New Vanilla Space Marines? There are about three units I'd never use in it; the rest are potentially fair game:
-Scout Bikers (utterly inferior to regular bikers)
-Vanguard Veterans (If one has a power weapon, it costs as much as an Assault Terminator. T4, 3+ guy with three S4 power weapons; or a T4 2+/5+ guy w/ Lightning Claws or 2+/3+ w/ two S8 attacks? You decide)
-Venerable Dreadnoughts (Just not worth the 50pt upgrade)
New IG? I'd forget about three units:
Everything thing else in the codex is workable, as a choice.
For the record, it's too soon for me to tell you about the SW codex. At the end of the day, my first impression is that it's just different from the SM 'dex.
Why Is This Important To You?
Well, part of this hobby/game is actually putting guys on the table. Not all codices are created equal, but I'll tell you that there's less mileage to get out of a codex with few options you really want to take to the table.
Your side (and the opponent's) of the game consists of three things:
1) You, the general
2) The Dice
3) The Army
You can always improve your game. The dice should theoretically be a random factor. The army, though? That's also on you. I maintain that GW will sell more models (which is a profit-driver for them, I'm certain) if they have good rules and codices that present us with a variety of options. Honestly, I think the last few that have come out have been a step in the right direction.
Also, Chaos Marine Folks? I feel for you in lack of options.
I'll admit there's a challenge in making the more limited codices work, but it's on you if you want it. Some people do it for the challenge, and some folks stick with a codex because they're fans of the race. I'll call on everyone to just respect people's motivations for picking their codex, as I think it's a variety of things that bring you to your army choice.
I hope GW keeps kicking out good codices like they have been. Players with options are happy players.