Time travel- we all have thought of it, or been exposed to some story where it features prominently. But is it really as safe/useful as it looks in the movies? Personally, I don't think so, and my main reason for thinking so might surprise you. But, there are many other important aspects to consider when you think about venturing into the past that you need to think about (and really discern if it's even worth it). I run over a few that I've seen in popular culture first, and then give my main reason why it's not a good idea.
Meeting Yourself: One of my favorite Van Damme movies, Timecop, hits this pretty hard. "The same matter cannot occupy the same space." I'm not sure if this is really a huge concern, but it is an interesting question. If you touched a previous version of yourself (say, you gave yourself a high five) would you turn into a double headed monster before your puddled into non-existence? Or, would the cells of your body no longer be "the same matter" when you met your previous self due to cellular turnover? I think the latter is more apt to be the real answer rather than the former, but it's still a concern any time traveler might want to keep in the back of their mind.
Unintended Consequences: How vital is that blade of grass in your backyard right now? How about ten million years from now? There is an episode of the Simpsons where Homer gets a time traveling toaster, and keeps altering the timeline in an attempt to fix the problems he caused. It's a hilarious vision of what could happen, but I think it's entirely valid. There are so many things to think about running around through time that changing even one could have terrible consequences. I mean, think about it. Changing anything alters the timeline, and this could mean ERASING the timeline that you are a part of, not just changing it. Cable might have a noble reason for wishing to snuff fireboy in Deadpool 2, but doing so might actually ERASE his wife and kids rather than save them (or cause them to become prisoners in something even worse than a concentration camp). Just because that teddy bear he's carrying is in great shape now, doesn't mean his family is there (or safe).
Getting Stuck: If you can go back in time, you may not be able to come forward. This idea ties into the previous one. You alter the timeline enough, and there is NOTHING to go back to. That future you're from? It very well might not exist, and if it doesn't exist, it may be impossible to go back. Reason? Well... From your new perspective in the "past," the future you're from isn't set, and so you can't go back. Does that make sense? Hopefully it does, and hopefully you're ok with being locked in the Cretaceous Period dodging predatory animals larger than you with no hope of ever coming home (and dying alone with the knowledge that you probably just doomed the human species completely).
But none of these will matter as far as I can tell because of one small problem with current time travel theory- space. Yup, space. Space and time may be interlinked, but that doesn't mean that going back in time will just take you to where you're standing fifty years previous. Picture this: Earth is moving through space around the sun, and that sun is moving through space around the center of the galaxy, and that galaxy is moving through the universe, and that universe is expanding. We really don't know how fast it's moving and in what direction. We can postulate and theorize, but the exact numbers? Forget about it. And even if we DID know the exact numbers, we still don't have a means of moving from one point in the universe to another instantaneously.
And therein lies the rub. Say you hop into your bonafide time machine, crank it back 2000 years to go see if Christ was real, and press "Send." You zip through a cosmic portal back into the past- and pop into cold, hard vacuum. Earth is 2000 years away, still spinning and spiraling through space to where you left the future, the events on her safe from your silliness because you are freezing and suffocating to death. You quickly hit the "Return" button, flash through that cosmic portal again- and pop into cold, hard vacuum in low Earth orbit. In the few seconds you were back in time, the Earth moved- and you moved- and those movements weren't together anymore. Even if you canceled your movement in space when you moved through time, you'd still have a problem- because Earth won't have stopped, and suddenly popping into existence in front of a mountain traveling at thousands of miles per hour isn't healthy.
It would be like hitting a pinhead without being able to see it, not know where exactly it is, while it's moving at 100 miles per hour, in an unspecified direction. In short, if you want to go back in time, and not die, you'd need to be Domino.