What put me off was some very faint, easily missed, grey on white lettering on the reverse of the letter they sent me. So far as I recall it said, by accepting a smart meter, I was agreeing to these hard to read changes in T&C. If they are still doing that, the changes are worth reading, even if a magnifying glass and bright torch are required.
One change that alarmed me iirc off was that they wanted to have the right to remotely disconnect me. No explanation was offered as to how that new term differed from their existing rights, but they must have had a reason for trying to make me agree to it.
I also seem to recall there may also have been some wording suggesting that I must keep the display panel in an operational state in a visible place. I’d have wanted the option to remove the batteries and consign it to the back of a cupboard if I found I wasn’t liking it.
Since I am deeply mistrustful of the energy network operators, I have so far refused to accept one and so far, been successful. But I fear that when an old ‘dumb’ meter reaches end of life, they may well be entitled to replace it with a smart one, your only option being to do without the gas or electric supply.