Good question...
The goal is to have different dropdecks for different maps and gamemodes...
if you play in organized groups and use tactics, almost every mech can do a good job.
But if you wanna play faction in solo queue, forget it. And you can be easily frustrated by seeing entire teams being relentless farmed by premade drops.
Not always, and casual teams working better than premades are not unheard... But it´s highly umprobable.
I´m not a big expert in facction, but in my oponion some handicaps you will finf again and again are:
1) Dont choose the correct assaults-
By example, an Old school Atlas can make a colossal impact defending Vitric Forge, but you will found it difficult to use in Boreal vault, or in any game mode based in long range weaponery. Maybe a ECM Stalker is great in polar and mediocre in manifold, vitric, etc... Chose a bad assault can be a 100 tons handicap for your team...
I tend to perfom better in medium range agile assaults... The classic Battlemaster (LrL or LPL versions) or the MRM/ml Zeus are specially versatile... But maybe is only my taste....
2) Lurms....
Lurms can be efective in well organized teams with narcers and spotters. But please forget it in faction pugs. Yes casually you can make a great score with them, but this is too much situational.
a) The clanners have a lot of ECM mechs. Blood Asps, Hellbringers, Sun spiders, Cheetas... They don´t sacrifice to much to bring ECM to the battlefield.

IS lurms have minimal range, take more space, have different firing patterns...
c)Clanners can put lots of damage at distance, even out of the lurms range... You can be losing your trades more fequently than gaining them.
d) Lurms take too much time in killing. A bad situation when you run against the clock.
3) Peek a boo against clanners...
In similar levels of skill, bad tactic.
In general Claners outrange IS, outdamage IS. Again, facing a competent player, you are not reliying in the two main advantages of IS: Better armor and in general better heath management.
Every time you play the poke game, you give the clanners the oportunity to take a favorable distance, a better position, and the time to refresh after devastating alphas...
Dot take it too literal, and don´t charge Leroy jenkins style, but if you see your team being not enough agressive, prepare yourself for a defeat...
3) Time to get the kill---
Builds that can be funny and useful in Solo Queue, can maybe work not in Faction... The enemy team can be reinforced if you need half a minute to kill a mech. Maybe you start a fight in a confortable 2 IS vs 1 Clanner... And find yourself alone ves 4 clanners... Take in account that clanners are faster. If your black widow hits 75 kph, their linebakers hit 104, and their hellbringers 84 or 87.
It seems evident, but the most part of the players dont realize that in faction, play the stronger IS points agains the weaker clanner points matters... They want to play like PUGS...
And this works only when the other team play like pugs too...XD