Hi! Thanks for the question.
When we first started drafting TnT, it was, by and large, a reskin of Sagas of Midgard's mechanics over the Cosmic Horror/Noir setting. But as we began making it, we started trying different ways to differentiate the two games. The dice pool idea was Dom's: he always liked getting to use all of the different dice in the set, and the dice pool/Assets and Liabilities mechanic was a way to get to "roll with friends".
It seems more complex than Sagas, and I suppose in some ways it is, but so far our playtesters have picked it up quickly and enjoyed it. It's also served as a reminder to us as designers that where you start is often not where you end up: while we've used certain parts of Sagas and ideas from it (Artifacts, for one, and the Raiding Mechanic became the "Pursuit" mechanic that we'll unveil in a later back update), Terrors and Tommyguns has grown into its own game with its own mechanics and, if we say so, it's a heck of a lot of fun.
Thanks for the question!