Have you tried it, or are you just imagining the worst? You usually take only half the damage you would be taking from hitting yourself with a rocket during a rocket jump, it doesn't eliminate damage completely, so you are still doing a good bit of damage to yourself if you try to go close-quarters rocketing. And you lose your shotgun, four rockets go really quickly, if you aren't hitting and running you are going to need your shotgun, those boots aren't worth it. The pickaxe, maybe, the new rocket launcher, as long as you have good aim, the flag, doubtful, maybe a few situations, but the boots? I wouldn't use them unless I was playing on some gimmick map that needs lots of rocket jumps.
I'm more worried about the demoman's new grenade launcher, that one seems to have no real downside. Seriously, the only weakness of a demoman's defense, aside from the possibility of the pyro compressed air thing or a spy, was that he only has one set, he blows those and he is out until he sets up more. The new launcher means he can't set grenades at a CP and then go to a different one, so what? Anyone that isn't a scout taking the last flag is going to make sure to scatter the grenades before they move on the CP, so it doesn't really matter. He can set 16 grenades now. One sticky grenade with a bit of luck kills a full health scout, spy, sniper, or engineer, two will kill anything without a health boost except for the heavy, those grenades are extremely powerful. And now he can set up a corridor of death with 4, 6, or 8 groups of them. That part won't require you to be a skilled demoman, just a patient camper.