I've had a Marcus online savings account, FDIC insured, for years. I don't recall any problems. I also have a very similar Synchrony account. No problems either. No charges at all including transfers. Decent interest. No minimum deposit. That and FDIC insurance are my main concerns. Throw in free transfers, beats the hell out of B&M banking where you get almost no interest.
I pay my bills via checking account autopay. If my checking account looks low, I transfer from online savings, free, via ACH. IIRC my online savings support free wire transfers, which I can do to my E*Trade investment account.
I've had other online savings account too. They can differ in some regards, you can always call them and discuss specifics. I don't recall any problems. I always make sure they are FDIC insurred.