The game doesn't favour "all comers" lists particularly well - it works better with dedicated tasks, and min-maxed points options. All lists need to cater for all eventualities, but looking at Eldar, whose units, by their very definition, are dedicated to one particular job, this problem is exacerbated and highlighted even more.
Take a look at it this way. Space Marines Tactical Squads are designed to be able to take on any foe the enemies of the Imperium can throw against them - equipped with Bolters, they're extremely effective, and with access to weapons like Lascannons and Missile Launchers, they can take down Monsters and Tanks, and special weapons like Plasma guns lets them be a massive threat even to things like Terminators, or Ork Nobz. That's fine, and perfect from a fluff perspective, similar to how soldiers for today are equipped with rifles, and have access to LMG's, and rockets to deal with harder threats.
However, from a gaming point of view, that "all comers" squad is wasting half of its potential. Without access to split firing, as can happen in real life, that Lascannon is wasted shooting at a horde of hormagaunts while the rest of the squad guns them down, or those bolters just ping harmlessly off the front armour of a land raider.
So, that means that tactical squad's strengths aren't really all that "tactical" or flexible, and can only really be built in one particular format - which takes us back to the days of 3rd and 4th edition, when squads were no larger than 5 men, and took Lascannon and Plasma gun to maximise that option. Now we're "incentivized" to take more marines due to the points/model tax, which channels them into min-maxing a particular squad into anti-infantry purely, which tends to limit them to Heavy Bolter or Missile Launcher (primarily using Frags, although it can, should opportunity arise, use Kraks, but the difficulty of penetrating and causing adequate damage is almost a nonentity), with a Flamer or Plasma. If they're targeting heavy armour they're doing it wrong.
So, on paper, they're flexible, but in the game meta, they're limited to anti-infantry duty, where their S4 AP5 Rapid Fire weapons are somewhat effective. Now, compare that to Eldar, who have units who are dedicated to anti-infantry - either as Dark Reapers, or Warp Spiders with Rending S6 Assault 2 and a Teleport to get close. Eldar do dedicated jobs better than units which are "balanced" to doing multi-task jobs.
If you need armour destroying in a Space Marine army, you have access to several options - usually Devastator Lascannon squads, TriPlas Predators, MeltaDreads in Pods, or Sternguards in squads with Combi-Melta. Compare that to Fire Dragons in a Wave Serpent with Scatter Lasers to ensure they get there. 5 Fire Dragons costs 110pts. For anti infantry, you have Warp Spiders at 190pts, which isn't far off the same cost as the tactical squad - but the Tac Squad is putting out 16 S4 AP5 shots, Flamer and a S4 AP5 Small Blast while the Warp Spiders are churning out 20 S6 Quasi-Rending shots for roughly the same cost.
Yes, there are other mitigating factors, but a combination of speed, low points costs, effective transport vehicles, and psychic powers means that Eldar are better doing dedicated tasks, and Marines can only hope to catch up.
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