@Jo Wo it may depend on the exercises. One that I did was lifting the leg and moving it outward so a sofa probably would not have enough room. You also want good support and my sofa has several cushions that tend to sink where they meet. But you won’t know until you try!
My surgeon, unlike most, doesn't allow any formal PT at all for the first month after a knee replacement. He says your knee needs that time, to start on its journey of healing. For that month, we rest, ice and elevate our leg, and walk around the house. The walking is our exercise and we increase it a little each week.
After that month, we just go to PT once every 2 weeks, where we are shown a few new exercises to do at home each day.
His patients all do well and achieve good ROM, as I did, and he hasn't had to do a manipulation to help with ROM for the past 4 years. I think that speaks for itself.
Personally I don't think a sofa would be ideal. Before I knew better I had PT in a spare bedroom. After learning about the BoneSmart way I was either in the den or the bedroom. My home PT learned on the first visit we would just do ADL's and possibly walk around the house. Of course she had a fit, but I explained I was literally paying her to "friend" me for an hour.
She wasn't comfortable doing it my way and I said kept saying its my knee, my house, my rules. She called her boss and my surgeon to tell on me!! Both said she was to do what I said!! She finally relaxed and those 6 weeks flew by. (My insurance mandates in home PT.)
A bed is good because you can lie down and get on and off of it from different directions depending on the knee you had done.
At various home PT appointments with each knee I was lying down, sitting in a (non-rolling) chair or walking a bit. (Not all of those at a time.) My home PT also helped me strategize and build my confidence going up and down stairs which I need to do for laundy.