My machines have been on for the past 12 or so years/ Basically they get turned off for working on them, or leaving for a longer period.

..not exactly proud of it, but i forgot to turn off the Duetto before a week long vacation one time. It was fine, just ran out of water and powered off.

I wouldnt leave a Silvia on 24/7, but any of the "prosumer" and up machines Id leave on no problem.

I would always check if various safeties are in place before leaving a machine on 24/7.
if a machine is plumbed and has auto fill etc there likely is little risk, f.e. my Urania does not and it has no safety other than an over pressure valve...coming home and finding it blew it's over pressure valve or the heater burst as water level was too low is not a great scenario.

I had a Lelit Mara a few years ago, only had it like under 6 months. Anyway I turned it on one morning morning while I was getting ready, Under 30 minutes everything started to see you next she like was going to explode. All of the water burst from the top and leaped out all over my counter, Now a little history about myself I've owned several machines Before & After and this has never happened again. Only thing I knew I paid $1800 for it when I returned it to the conarctis seller, He gave me $600 plus I had to pay for shipping. My worst experience ever since taking on the espresso hobby.

I have a La Scala 'Butterfly' that has been on at 8 am and off at 10 pm generally. The machine isn't plumbed in so leaks aren't a problem. One issue I have had during its life is failed pressurestats (Mater xp110). I have probably replaced 4 of them over the last ten years or so and am currently replacing a unit that lasted a month from new before going over pressure at irregular intervals. Unfortunately, I don't trust leaving it on when I am out any more. Luckily, it doesn't take too long to get up to temp from cold and I guess it saves a bit on power as well.

Machine on @ sunrise, off at dinner.
365 * 30+ machine years [multiple machines].
No issues.

Seems like a lot of Lelit machines have issues
Could be sales volume
Or QC