Clan and IS are pretty well balanced. In the first moment you see Mechs withs huge advantages like Clan XL Engines, superlong ranges and double damage autocannons as well as less tonnage for the same armament (e.g. LRMs).
But IS mechs have Quirks to compensate and much higher precision on every weapon. Low laser durations and heat efficient lasers and PPCs, one bullet per AC instead of multiple bullets spreading across multiple components and guided SRMs that spread so hard, the are almost useless vs. everything except light mechs. And their LRMs come in a stream of missiles instead of a single volley, so when lock breaks they only can apply a fraction of the damage and can be easily countered by AMS. Clan PPCs lack velocity quirks most of the time making them inaccurate at higher ranges.
If you can use that precision to your advantage, make use of your mobility (although Clanmechs usually have higher top speed but lack agility) and structure quirks, IS-Mechs are at least on par with Clan Mechs.
Oh and most players struggle to build good loadouts in the mechlab. Clan Omnimechs are more easy to build and more beginner friendly to drive because of Clan XL.
I don't feel like playing the "bad mechs" when dropping in CW with my unit or fighting in solo/group queue. Often I feel sorry for the clan mechs because of their disadvantages
But I also play clan mechs very often so I know where the strengths and weaknesses lie.
Edit: Oh and there is the "its more expensive, so it has to be better"-factor that applies to both clan mechs and assault mechs. It's just a wrong assumption.
Edited by Exard3k, 11 October 2016 - 09:53 AM.