Once you get in the later parts of the game, the tech points add up a lot, especially in the last few "dungeon" areas, and that wont be an issue anymore. The first game probably does have the weakest combat system of the three, but the story more than makes up for it. If I remember correctly there are specific weaknesses for each class, I might be getting confused with II or III though, maybe an FAQ will answer that question better. In terms of comparing the first one to Xenogears, the battle system in Xenogears is much much better, but both games have excellent stories, none of it feels too rushed either since they expanded a large story in a trilogy instead of one game. Once you get through the first one, the battle system gets much better in Episode II and becomes the great battle system that is in Episode III. That was one of my few complaints with the first one though, the combat system is too simple and easy, that and the spacing of the cutscenes. The developers improved on both of those aspects in the next two games. Trust me, if you like Xenogears and its story you will definetely enjoy these games, if not the combat, you will love the story.
EDIT: If I remember correctly Gnosis are weak against some ether and beam attacks, Mechanical enemies are weak against lightning ether, and Biological are weak against physical attacks and sometimes ether. It changes up here and there especially with bosses but you'll start noticing patterns in weaknesses and battles will go much quicker, just try it out and experiment. There are some spoiler free FAQs on GameFAQs that might give more detail though.
