Does it matter what faction Mech World is?
It matters for the solo variants.  I can't find rules for these online but I can explain how they work in the app.

In a typical solo scenario on the bots turn a card is turned up and depending on the faction this dictate how it behaves.  This image is from Star Realms website to explain show what I mean and is best 'reference' I can fine.



In the app there is a weekly scenario in online 'Arena'. The rules for these are different each week but I have seen games with a random faction removed or different factions.

It also matters if playing with Missions expansions.  Some of those cards will refer to playing cards of the same faction type.  That is not the same as triggering an ally ability of all factions..

There may be other answers to this but there are lots of expansions to look at!