I'd say the exchanges that currently remained to be unhacked are pretty "safe"; probably Binance and Coinbase. But that doesn't mean I'll recommend leaving funds on these platforms. The developers can make the platform as secure as possible, but never secure enough to the point that it's impossible to hack. Exploits will always be a risk. Not because these "unhacked" exchanges remained unhacked for years doesn't automatically mean it'll stay the same forever.
https://NotYourKeys.org How do you explain these things to the people whom have entrusted their funds and assets to you for safe keeping hoping that what they gave to you is under safe care only to wake up to a sad news of their assets being stolen by hackers. How do you explain all these.
That's why you don't let other people entrust their funds to you. You have not much to gain but everything to lose.
Funny that you mentioned unhacked, these two remaining centralized exchanges receive hacks and security breaches amounting to millions of dollars for breakfast. So all things considered they are pretty vulnerable to say the least. However, what assures me that these two exchanges are secure enough is how they remain operational even after massive attacks like this. FTX received two consecutive hacks and they literally broke down, Binance receives hacks and even then they remain operational, with a few times of contingency plan initiations but never to the extent of what happened to FTX. So when choosing your exchange, make sure you find one that could save your assets when push comes to shove, although of course, at the end of the day always consider owning a self-custodial wallet for maximum security.