Buying creates new fund shares and selling cancels them, with the fund transacting in the underlying securities to match. Every order placed before the cutoff gets the same price: that day's nav. There is no intraday price, no bid-ask spread on the fund itself, and no ability to use a limit-order.
The structural weakness is tax. When other holders redeem, the fund may have to sell appreciated positions and distribute the gains to everyone still invested, including someone who bought recently. An etf largely avoids this through in-kind-redemption.
Example: you place a $10,000 buy at 11:00. It fills at the 16:00 NAV of $32.46, giving 308.07 shares. Had the market rallied 2% after 11:00, you paid the higher price.
Related: nav, etf, index-fund, in-kind-redemption, expense-ratio