The SaGa series is the brainchild of Akitoshi Kawazu. He was an early designer for the Final Fantasy series. He played a large role in the development of FFII since Hironobu Sakaguchi was moved from designer to over-all director for the project. As a result FFII is a much harder game with an emphasis on realism, particularly in the way characters level up. The ideas he had during the creation of FFII would carry over into the SaGa series.
SaGa Frontier was a great game but was very difficult. Kawazu's emphasis on realism meant that from the beginning of the game your character could go just about anywhere, and with no scaling of enemy level this meant that you could start the game, go to an area, and immediately be killed in one hit. Characters didn't have levels, instead attributes that were used the most got stronger (just like FFII). New powers could be bought but were also randomly learned when similar powers were used. The big drawback of the game is that it could be hard to know what to do next. There was no quest log or way point system like modern RPGs.
Over all it was a good game. It was split up between several characters, and you could play their stories in any order because they all happened at the same time! You could even get other main characters in your party before you played them. What's interesting is that many of the ideas Kawazu had later became staples of western RPGs. Think about a game like the Elder Scrolls. It's an open world were you can go anywhere. Your stats improve as you use them. The choices you make mean something. It's ironic that a Japanese designer was already rebelling against the tenets of what would become JRPGs at the very dawn of the genre. Had Kawazu had a more enduring legacy with the Final Fantasy series the RPG landscape could be very different today.
I didn't play very much of SaGa Frontier 2 but I liked what I did play. It was a little more polished and organized than the first one.
Unlimited SaGa was a disaster. Everything about it was incomprehensible. The battle system had some sort of spinning ring mini-game. The world navigation was a sort of game board where every space had some way to kill you. I knew it had terrible reviews but I bought it anyway because it was 3 dollars used. I thought I'd give it a chance because the Frontier games were underrated. No. No. It really was god-awful.