A thought recurs, more or less every time I run through a new list for my cadre; don't gild the lily. Gilding the lily, for those who aren't familiar with the saying, came out of Shakespeare's King John. It's actually quite a contracted paraphrase of Shakespeare's line, which advises against gilding refined gold and painting the lily, amongst other things, on the grounds that such an action is "wasteful and ridiculous excess." With regards to 40K, the danger of gilding the lily is in pouring points into a unit, either to make it somewhat less mediocre at a task for an unreasonable cost, or to make it better at a task it's already excellent at, even for a small cost.
Kirby, of 3++ fame, recently wrote about this same subject. But as good as Kirby and his crew (krew?) are, their wide audience requires them to talk in generalities; as an exclusively-Tau Empire player with a, let's just say less extensive audience, I can be more specific. Because the issue of gilded lilies is both more subtle and more dangerous for adherents to the Greater Good than for most other codices. The issue is that Tau Empire cadres, through their XV8s and to a lesser extent the '88s and vehicles, have the kind of customization options that, say, Nob Bikerz get access to. But where Nob Bikerz can really take advantage of their customizability to increase their threat value, mostly through their wound allocation shenanigans and their ability to get both cover saves and FNP, the dropoff for Tau units is both much earlier and much sharper.
So, as we all know, XV8s get three options, usually used for two weapons and a support system. But that's just the standard ones; team leaders, shas'vres and the army's shas'els and 'os get access to the special wargear section, which contains several duplicates from the support systems section, like multi-trackers, target locks, drone controllers and blacksun filters, along with unique pieces like bonding knives and the special issue items. It can be extremely tempting to bump a member of an elite XV8 trinary up to a team leader or shas'vre, and use them to add something to the squad. Targeting array, target lock, shield generator, drone controller with shield drones or networked marker drones, there are a variety of options, and in and of themselves, they're not bad options. But they're not cheap options, either, and they almost never manage to recoup their investments. A team leader with a targeting array is an extra fifteen points; with a shield drone it's twenty, thirty-five if you want two of them; twenty-five with that shield generator. And these aren't cheap squads to start with, with three fireknives clocking in at nearly two hundred points for just three models, with no invulnerable saves, who can easily be one-shotted by the plethora of S8 weapons running around the game at the moment.
And the XV8s aren't the only lilies out there tempting you to gild them, either. Tau vehicles have a tonne of upgrades that look good at first; SMS, targeting array, disruption pod, decoy launcher, multi-tracker, flechette discharger, they're all pretty good looking. And given the rather lacklustre performance of most Tau vehicles, it's particularly tempting to start throwing things on their to increase their effectiveness. But a naked Devilfish is 80 points, hardly a cheap transport to start with, and the costs climb a lot faster than the value of those upgrades. The disruption pod is basically a must-buy, so the Devilfish arguably costs 85 points, and if you add the SMS then, well, you should really add a targeting array and a multi-tracker to get the best out of it, and if you're going to be operating that aggressively some flechette dischargers wouldn't hurt, either. And before you know it, you have a tank that's not especially more difficult to kill, and hasn't particularly upped its damage abilities (with a multi-tracker, the SMS gives you two extra shots moving 6", and one fewer moving 12"), but suddenly costs fifty more points. And again, most of what you've added are the exact same S5 AP5 shots you can find every last Fire Warrior toting, only they've got better range, a higher rate of fire inside 12", and for those fifty points you actually get one more shot than the SMS offers at beyond 13".
The Tau Empire codex is one of the most option-heavy going, in terms of pure customization opportunities. Battlesuits have more unique options available than nearly any other unit in 40K, and Tau vehicles have some of the best wargear upgrades available, though sadly that's balanced out by their mostly having some of the worst weapon options in the game. But as I have learned, through rather unpleasant personal experience, the Tau also have some of the sharpest levels of diminishing returns going. The army is, ultimately, a force that has no meaningful close combat abilities, average shooting power and defenses, and weak leadership. Rather than relying on a few heavily upgraded units to carry the rest of the army, it's terribly important for a Tau player to remember that old Soviet cliche; quantity has a quality all its own. The more saves you can force your opponent to make, even 3+ or 2+ saves, the more saves they're likely to fail; conversely, your opponent will pass every save you don't force him to take. The same is true for most other races, with variations, of course. Despite its origins, 40K is currently meant to be a game of armies, not heroes, so bringing a few incredible heroes and the bare minimum of an army along often won't end well for any army commander.
After all, you can't do much fighting with nothing but a few lilies covered in yellow paint.