This is a mistake you'll see newer players make a lot, but even for more experienced gamers it's hardly uncommon. It usually starts something like this:
"Man, Draigo and a squad of Paladins is just about impossible to kill; that's a really strong unit and it can support itself with shooting. Give them all Halberds or Daemonhammers and you're looking at a really tough squad to bring down. Oh man, and with a Librarian I can mix up weapons for wound allocation more and have 3+ cover and S7, +2D6 armor pen, I10 and other good stuff. And if I add a Techmarine I can get Rad and Psychotroke Grenades also so I can Instant Death almost anything and kill even big Ork hordes! Man oh man this unit is so unbeatable!"
And occasionally it will be, but against a decent/good opposing general it's actually very underwhelming because you're falling into the "win-more" fallacy.
The fallacy, essentially, is continuing to pay points (or other resources, depending on the game) for things that don't help much when you're losing, but when you're winning help you win even more- which is generally unnecessary and a poor use of those points. In the above example, Draigo + buddies should already murder virtually anything in close combat; as you continue to stack things onto them, the improvements become more and more marginal because there are already so few units that stand any chance against you in a fight. You could have instead spent those extra points to shore up weak points- say, by adding more scoring units, or more long-range shooting, or whatever, which are all very real weaknesses of the list. Using your resources where you need them is the fundament of good list-building.
Although such melee "deathstars" are the most egregious offenders in this category, they are far from the only one. "Spam" lists often fall prey to this same problem of one-dimensionality; if you've already got six squads with 3+ Meltaguns in them, do you really need another, or do you need some more long-range firepower instead? Is that 24th Lance weapon really going to tip the balance, or might you be better off diversifying a bit? This is the counterpoint to saturation and part of a strategy of flexibility- as you stack more and more of the same thing up, they often become less useful. Finding this balancing point is very important and there is no hard and fast rule for doing so; it will vary with each different army and each different list. What is overkill for one might be insufficient for another, but it's importan to understand that this point exists, because there is often the perception that more is always better, which is patently untrue.
"But of course being better at something is good!" some of you will be thinking. "That way even if I lose X members of the squad, I can still kill Y!" I'm sorry, no. For one, in almost all cases you would be better served buying another squad to assist you- "Boyz, not toyz" as the saying goes. Second, overkill can be very harmful to you- charging a squad and completely wiping it out can often leave you exposed and in the open, so limiting your kill potential is actually a good survival strategy. Last but not least, expecting a single unit (or type of unit) to handle all your army's problems is, at best, a dangerous gamble and at worst outright foolishness.
The lesson to take for this is not to just keep throwing points at something until every possible problem is solved; EVERY unit has weaknesses, and you can't fix that. Terminators hate low-AP fire; Assault Terminators dislike tarpits; everything has one or more units they just can't handle, so accept it and take something else in your army to deal with that threat. This isn't to say that you shouldn't try and cover your weaknesses, but you need to be aware of how many points you're spending and to what degree the weakness is an issue and will be mitigated by your countermeasure.
