Just casually reading r/fidelity on Reddit (which is a subreddit run by Fidelity) would make me run fast and far from using their cash management account for anything. Widespread check fraud has caused Fidelity to be extremely cautious -- read slow -- in giving people access to checks they deposit into their CMAs. I'm not saying this is a bad thing, at least Fidelity is being cautious about taking care of their customers' money, but it's created a good deal of pain and anger for people who are depending on Fidelity for everyday banking-style transactions. I'll invest with Fidelity, but I prefer to keep my everyday money in a traditional bank or credit union.

if you have investments at fidelity you can just use the brokerage account as cash management. the brokerage account can access funds using a debit card and ACH just fine; I do still recommend opening a CMA for ATM fee reimbursements though.

none of these funds availability problems can happen if you have margin enabled; note that you won't pay any margin interest either as funds on hold are tradable immediately.