I didn't find them relaxing at all.
I don't ever find much of an opportunity to "relax" when riding, got to be on my toes!
The main benefit of the forward pegs for me is the ability to straighten out my old knees once in a while.
Brings more weight on your bottom
Agreed. Recently rode a friends brand-new Honda Shadow with forward controls. Painfully learned that I will never have a motorcycle with forward pegs only, since there is no way to relieve the weight from your backside. I like having the pegs under me so I can "get up on the pegs" when the pavement gets rough. Low back injuries are cumulative, and I've accumulated a couple of minor strains.
... you are less in control of your bike.
They do require practice, just like anything else. I typically move my feet back to control pegs when in heavy traffic, approaching an intersection, etc... I have occasionally found myself reaching a toe out for the shifter and/or brake with my feet on the forward pegs.
I just need more practice!