Time travel is hard to do right. This show tried and failed, not always miserably.
If you have a good memory and cannot stop doing basic reasoning on what you hear or see, you will not enjoy it.
We follow some agents from the future that are here to save humanity. (No pressure)
They have to follow some rules that make more or less sense and are sworn to obey the intelligence behind the great plan.
The intelligence itself has to follow some other rules (because it's "the good guys") and sending somebody back can only be done under some specific conditions. For example it has to know the exact position a few seconds before the time of death of the recipient, It can never use somebody that was not about to die. (Of course there rules are broken when it suits the plot, and coherence be damned)
In almost every single episode, the champions of humanity are acting like your everyday naive idiot. An example ? A member of the team gets some medication from some very suspect woman to keep him from hard drugs, allegedly. The MD will barely raise and eyebrow and days later, after the "cured" guy acts like even more like a violent junkie looking for its next fix, the "wise guy" of the team, the oldest soul, will only make a comment to the crazy teammate but nothing more will be done to get to the bottom of what looks like some very dangerous and very suspect situation. (Knowing they have enemies)
There is a made-up rule that say one cannot send a traveler earlier that the last one that was sent because of some ripple effect. Which is nonsense (even in this context) as the future is allegedly always fixing by incremental changes and can kill any adult by making it a phone (making a possible problem disappear). It is clear this was though-up to induce a weakness in the plan, and this is actually said in one episode "we get one go at this, there is no do-over"
Speaking of which. The arch-nemesis of the serie is an insult to the intelligence of the viewer. The first traveler ever refused to kill himself (because, reasons, don't even try to make sense of it). The guy was on one of the tower of 9/11 and we have to believe him exiting the building alive was not caught on the always spying eye of the future ? That in the years that followed he never was close to a device that could spot him ?
Killing his wife making her a messenger to basically ask him to kill himself was a better plan than, say, sending somebody inside her (still killing her, mind) to
do the job ?
But given the number of stupid mistakes this Deus Ex Machina does (so something bad can happen, lazy storytelling 101 a.k.a Macguffin) this is not really a surprise the spectator can come up with a better solution in a couple of seconds while hanging the laundry ...
I could go on and on and on ... and this is only about the laziness of the writing.
I will barely mention the situation where a tiny skinny girl beating up a group of violent guys (maybe they were particularly ticklish) and other accidental comical situations.
The show is not all bad though. Sometimes, every few episodes, there is some scene that made sense and looked properly prepared to unravel slowly. Some scene that would make me smile. It was always ruined in the next five minutes though, but still.
I don't have anything bad to say on the acting, the effects or the music so they are probably adequate, at the very least (not that I had my eyes on the TV : I've endured this to have some "noise" in the house while doing various chores
The one show that I've seen completely was the last episode of season two. Probably one of the worst episodes when it comes to coherence and rationality. I may not be the most feeling-oriented guy on the planet and may be surprised by reactions based on them, but what happened there was just ... artificial, irrational, uncalled for. And of course you will have to forget all that was said for two seasons stop yourself from screaming at the screen.
With these champions, if they ever do a third season, I foresee a tragic ending.