I'm a Junkrat noob, so bare with me, but Junkrat has a few problems that I can tell:
They think he's easy cuz no aiming. But positioning and enemy priority is so important with him. Most people play him terribly. They aren't aggressive enough with him or they dunno how to use his abilities correctly. I saw a huge jump in Junk usage after non self-damage was patched in and the extra amount of bad Junks skyrocketed.
First is that he's one of the most easily counterable heroes in the game in a fight. I feel like I could take him down with almost any character because his gun sucks for any close range encounter, but is also only kinda effective at mid-distance. Sorta. His gun is actually strong if he hits you, but his projectiles are some of the easiest to dodge in the game and he's meant to be constantly spamming them in general areas, so it's easy to catch him reloading. And I guess people kinda don't learn to aim with him because he's spammy. But in my experience, whether I'm playing Genji or Tracer or D.Va or Zenyatta, he's always just an easy target that rarely seems capable of firing back on me with any reliable success. I'm not saying his gun needs a buff (Well, not in projectile speed or damage anyway. A larger clip size would be nice though), but it's the reason why everytime the enemy has a Junkrat, my immediate thought is "Cool, free kill!" because even good Junkrats can't seem to actually fight people off. His weapon projectile is just too slow.
Second, he feels like a counter dive that can't do his job. It's something I never realized until I started playing him, but his kit actually involves a lot of mindgames predicting where the enemy will be while hiding his traps enough that savvy players won't spot them. I've had a lot of success putting traps not at choke points, but right to the side where chokepoints are, because players check to see if there is a trap in the immediate area but not anywhere else. However, he has one trap that is on a kind of long cooldown and it takes a long time to set up in the heat of battle. This means that even if he catches and kills one person....well, it's not nothing, but it's his one trick and it's used up for the entirety of that battle. I've been saying he should be able to set up 2 or 3 traps at once, which would help a lot without pushing him into overpowered territory.
Third, because of the first point, he has a distinct lack of escape options. His Concussion mine lets him be mobile, but it means he's making a sacrifice. He can either use it to make himself fly (which, btw, you can fuck up if you detonate at the wrong angle) or he can use it to commit to a fight and hopefully kill the person attacking you. It's a good ability and I don't think this needs to be buffed, but it's a costly one, which means if you fuck up, you're pretty much screwed.
Also, his ultimate is the worst in the game. I don't think this is a very debatable point. it can be clutch in some circumstantial situations (not too long ago, I saved a point by myself with it), but overall, it's very high risk for relatively low reward.
I've had success with Junkrat being a backline fighter that spams grenades, preferably from high ground, and playing general mind games with the enemy. I have to anticipate to how they move before the battle even starts and lay down traps to cripple their push and that means that there is a large degree of the battle outside of my control. I'm a hunter lying in wait, counting on my knowledge of maps and enemies mindset to get kills, which is a fun and satisfying way to play, but it also means there is just less I can account for vs playing Genji, who actively hunts other players down. And if the enemy decide to react to me in particular, well, there's only so much I can do from there.