It really depends on the horse.
The first one I ground trained was a wild, idiotic little nitwit who's now an in-your-back-pocket-all-the-time loverboy. But, that doesn't mean he loads, bathes, or picks up his feet well. Very friendly, but has barriers that need to be knocked down. On a training scale of 1 being the easiest, and 10 being the hardest, I'll give him an average of 5. He's not an unusual case, and what one can expect to find in a unhandled 2 year old (now four).
On the other hand, I have Divo who picks up on things faster than I can teach them. He's easy to teach new things and is accepting of new experiences. On a training scale, I give him a 2. He's is unusually easy to teach, but does need some practice as to not develop bad habits. His major vice is that you have to really move through your training list to keep up with his learning.
Then theres my colt, haltered twice and otherwise just not messed with much. He's about a year in a half, approaching two, but very personable. On a training scale, just from what I've tested with him, he gets a 4. He'll be easier to train than your average unhandled horse, but he will have a stubborn streak and at the moment has a short attention span.
All in all, is training easy? No. Definitely not. It's an extraordinarily dangerous profession. Even with Divo, my unusually easy learner, I might hit on something that will make him blow up or buck. Maybe he doesn't like tarps, or maybe he hates water. Theres always a roadblock to overcome on an unfinished horse.
I have to say that finishing a horse is one of those difficult tasks one could ever undertake. There are so many different things to be covered, and each horse requires some slight changes to the program than the last. Maybe they need more pressure, less pressure, more of a gentle touch versus a firm hand.
I always suggest starting with a trained, not exactly finished, horse. Something with some small barriers to overcome. The more horses your around, the more confident you will become. Eventually you'll come across one that you just can't resist taking home and training yourself.
Wait! I'll fix it....