Have just reponded to your other thread and said some of it already lol
Partly because I found her too big to ride,very wide was hell on my hips and pelvis after half an hour or so.I also had hernia surgery a few months ago which didn't help,she was just too big for me bless her.
Also heavies although kind and placid are also very clever,and can be a pain of they learn how to use that size and strength against you.I never had too many problems like this to be fair with my girl,but I know other who have,and mine could have the odd 'moment'.something that weighs half a ton if it minds to can be very intimidating!!
Another huge part of deciding to sell her though was the fitness issue.A horse this size needs a LOT of work,little and often to keep it fit.It's all or nothing with these types.Took nearly two months of my mare being ridden 4-5times a week to be able to canter a couple laps of the school,if I didn't keep this routine up for a couple of weeks her fitness dropped right of again and she would be huffing and puffing just trotting up hill!!.So basically if you just want to walk around the lanes once week,then you will get away with doing just that,if you want to do anymore than that you will need to put in the work!! After talking to other heavy horse people,this is quite normal for a heavy,they just take a lot more getting and keeping fit.That's probably why so many people cross breed,adding something of a lighter build gives them that bit more agility and get up and go.
The new owners of my mare ride her every day,as they have found this the best way to keep her in shape.I could never ride more than a couple of times a week,and I found the pressue of having to ride made it more of a chore than a pleasure,ended up paying my YO to ride her just to maintain her fitness!!
The other factor would be practicality.Difficult to get stuff to fit (rugs were a nightmare,and never managed to get a noseband that fitted in the whole time I had her

),I had to have a lorry as transport as they don't fit in trailers very well

.Everything is more expensive,wormers (had to buy two lot's as the weight limits didn't come close!!),vet treatment (again had to have double dose of antibiotics due to the weight).Am very glad my was not shod as you don't even want to know how much a set of shoes for a heavy are

.All those kind of things just made life difficult.
I was very fond of my girl,don't get me wrong,but it was hard work owning her,and not because she was difficult,as to be fair she was a doll,just things you don't have to think of with a 'normal' horse can become a real problem with a heavy type.
I hope you find one if that's what you really want,but please speak to some other heavy horse owners,and do some research yourself first.I think they are magnificent beasts for what they are meant for (working,showing etc),but as a general riding horse,there are easier and more suitable breeds and types to own.