People often say that a certain plant is easy to grow, or difficult to grow. I say it's BS. Supposedly a great easy to grow plant that even a kid can plant is sunflowers. Well, I have planted mammoth sunflowers many many years and they've always either failed to sprout at all, or grown the saddest spindliest flowers I've ever seen. Last summer however my 3 year old son was helping in the garden and I gave him a couple sunflower seeds to bury just so he could plant some seeds that were large and easy to manage. His two sunflowers topped 12 feet in height, they also put out flower heads that were a good 12 inches in diameter. Maybe it was just better soil? Nope, I put in a few sunflowers too. Planted them at the same time and they only grew to about 6 feet and had 6-8 inch flowers. I have no idea why, but the sunflowers loved being planted by the small boy.
Among the so-called "easy to grow" plants most people list perennial herbs, tomatoes, pumpkins, summer squash, annual flowers, and hardy greens like kale and chard. If you are just starting out with gardening give these plants a shot, but don't give up hope if they don't do well. Sometimes it will just turn out that you have a knack for celery (known to be a very finicky plant), while tomatoes fiercely dislike you.
My brother built a massive greenhouse and garden last summer, on my parents' property. The garden there has been producing piles of tomatoes and summer squash for nearly 20 years. He saved the hot crops for the greenhouse, but continued the tradition of putting summer squash in the garden, he also planted peas, turnips, carrots, kale, broccoli, and chard. While this was the first year he managed such a large garden he is an experienced gardener. Guess what? Everything in the garden did great... except the squash, and the peas. For some crazy reason last year the only things that would grow well in that garden were root veggies and brassicas.
Some crops are considered easy because they have few known diseases or predators, or because they can withstand some drought or flooding. Very few veggies should be considered easy to grow, nearly all of the vegetables people plant are so carefully bred that they are entirely dependent on human kindness for survival. Kale is one of the few that could probably manage on it's own, and in hot climates it is challenging to grow outside of the winter months.
Often when you have difficulty with a certain plant it is not because the plant is difficult but because you don't yet have an understanding of what it takes to make that plant happy. Since a pumpkin makes a very small root compared to the massive amount of aboveground space it takes up, it doesn't generally need a lot of well worked soil. The pumpkins roots won't spread much, they can be put in a 6" pot in a hole full of rocks and be content so long as their 6" of dirt is highly nutritious. Meanwhile, carrots sprout from very tiny seed that then sends down a tap root, while carrots don't actually bulb like an onion (so they can be spaced fairly close) they are rather delicate and need their soil loosened and fluffed to make growth straight down easier. If you've got heavy soil and only loosen the top inch or so, your carrots will never do well. I have never yet been able to grow carrots well. This year I plan to try again in the "Mel's Mix" soil, in hopes that it is light and fluffy enough to keep carrots happy.
Once you get a handle on what the plant needs to be happy you will find it easier to provide those things and have a very "easy" plant to grow. For me, one of the "easy" plants I struggled with was pumpkins. I put them in the ground and they got powdery mildew, or didn't sprout, or didn't grow beyond a sprout, or grew long and happy and healthy but didn't produce any pumpkins. Each year I learned a few things so that I got a little closer to pumpkin perfection. Initially I had it in my head that pumpkins were colder weather plants, it seemed reasonable since they aren't harvested until fall, I planted them in March and my seeds didn't sprout. The variety I was growing needed about 100 days to reach harvest, so I figured out that I needed to start them indoors to keep them warm until the weather improved, this also helped prevent the mildew that sets in during the wet months in spring. Once I had a good start going and the weather improved I moved it into the garden bed, but that year I let them sprawl and only had room for one plant: it grew well but no pumpkins developed. Finally I began trellising, it gave me enough room to grow three pumpkins in my small space and finally had the cross-pollination needed to produce some very healthy fruits.
Gardening is a learning experience, each year you find that you get a little better at managing the plants you grow and get a little better fruit or flower in return. It's a struggle at first for most people, but it does get better. You may never be able to get the hang of sunflowers, but you may grow the best turnips in the county instead. There is no such thing as an "easy" or "difficult" plant, it all depends on the gardener, the garden, the weather, and how many butterflies flap their wings in Taiwan. Gardening is truly an experiment in chaos theory unlike any other.