When you look at fees, think "compared to what?". For HSA Bank, how much would you pay to switch $5,000 cash into $5,000 invested? Their fee schedule (for individuals, not employer accounts) lists $5.50/mo ($3/mo invest fee + $2.50/mo min balance fee) or $66/year. You can pay $0 fee and hold $5000 in cash, or you can pay $66/year in fees and invest all $5000 in investments.
$66 / $5000 = 1.3%. So if, like me, you think the stock market will return an average of more than 1.3% per year, you're better off paying the $66/year and switching your last $5000 to investments. This assumes an individual with an HSA, rather than an employer with everything at HSA Bank.