can they be hacked, and the funds stolen, do they have a special wallet where they keep all the btc ? how do they work exactly?
can they suddenly decide to hold your btc, or restrict you or prevent you from sending or receiving bitcoin ? if you don't provide id or for whatever reason ?
do you own your btc (menmonic) with them or is it in their control?
Online services can be "hacked" at anytime. That is why its most recommended people have control of their own wallets, instead of relying on a third party that might or might not do a good job protecting it.
Well managed online services will take a close look at how much of the funds in their possession are actually moving, this way they can move the large non moving coins into cold wallets, precisely to protect from such attacks.
But still, chances are the company might go bankrupt or will never be able to respond to all customers such an event like this occurs.
For some of the people replying on this thread: Please do not confuse blockchain.com online wallet service with Bitcoin blockchain. Blockchain does have a Bitcoin blockchain browser, but that's all it does, browse the blockchain. This can be done from many online sites, its read only, the blockchain is public and immutable. It can only grow, and to grow you have to find the next block, that is, mine it... I don't think i have to explain you how much hashing power you need to be able to find the next block and earn 12.5₿...