The Flash's time travel rules have never been entirely consistent - but with season 5, they've stopped making sense at all. Because time travel only exists in the real world as a theoretical possibility, every franchise that dabbles with it has to make some important decisions; just how does it work within their narrative? Can the past be changed, do you go with a multiverse model, and how do you resolve the various potential paradoxes? Unfortunately, the longer a story dealing with time travel runs, the more likely it is that the writers will decide to change the rules in order to open up new storytelling opportunities. That usually leads to contradictions and logic gaps.
Time travel has been a central concept in The Flash ever since season 1, and logic-wise it's just about worked, presenting the idea that a super-speedster can travel back in time and change the past, but that they have no idea what the impact will be. In season 3, Barry traveled back to prevent his mother's death and unwittingly created the "Flashpoint" timeline; the entire season essentially explored the repercussions of that one decision.
The Flash season 5 has brought back Barry Allen and Iris West's daughter from the future, Nora, a super-speedster who operates under the codename XS. The writers want Nora to stick around, so they've changed the laws of time a little. According to The Flash season 5 premiere, some points in time are more flexible than others. The death of Barry's mother was important to the timeline, so saving her rewrote history; but we're expected to believe Nora's presence won't change her time at all, and that she can stay with her parents without there being any cost.
Hw does a character know whether or not a given moment in time is fixed? You need a perfect knowledge of history in order to work it out; a time-traveler who wasn't aware Barry Allen is the Flash, for example, would never imagine that his mother's death was important to the timeline. Team Flash don't have enough knowledge of the future to possibly know whether or not it's safe to keep Nora around. Even Nora doesn't; as we saw in "Blocked", she had no idea her father had ever been in prison. The fact that Nora's knowledge of the past is limited means she has no way of knowing whether or not she's changed her own time beyond all recognition. Nor can she possibly say for certain that the changes she's made are for the better. Even allowing for this new theory of temporal mechanics, that some points in time are flexible, keeping Nora around just isn't wise at all.
The worst-case scenario is that Nora could be erased from existence altogether. All we need is for the Flash to be training Nora on a night when, in the original timeline, he and Iris were spending some quality time together. As a result, Iris never gets pregnant, and Nora is never born in the first place. One ill-timed training session would rewrite history and, crucially, nobody would know it until it was too late.
Then we come to the second major problem with The Flash season 5. Nora has come back to this time because she wants to get to know her dad, and she's revealed that the Flash is destined to mysteriously disappear. Iris is determined to change that, and even Barry is with her on this; he wants to be there for all his daughter's "firsts," after all. Unfortunately, nobody has stopped to ask what the cost of changing the Flash's personal future would be. The absence of her father is a foundational part of Nora's very identity. Should Team Flash succeed, Nora's personal history will be rewritten to the extent that the girl we've met in season 5 might as well have ceased to exist. No doubt Nora herself would be fine with that, preferring a timeline in which she'd grown up with a dad, but that idea should really be giving Barry and Iris sleepless nights. The closer they get to Nora, the more they come to accept her as family and love her as their daughter, the heavier the idea of changing time should weigh upon their shoulders.
It's easy to see why the writers have changed their approach to time travel; they actually want to use Nora as a character, and that means they have to switch things up a little. But it will be interesting to see whether or not The Flash ever acknowledges the problems with its new concept.