This certainly gives you more options and more financial safety, but if you keep this fund liquid (as emergency funds are supposed to be), you'll pay a hefty opportunity cost because that large sum will, at best, keep up with inflation, and at worst will have a negative inflation-adjusted return.

An emergency fund should be large enough to cover the most likely emergencies but small enough to avoid excessive opportunity cost. In most cases, you're better of with just enough than with too much.

You can always dip into investments if your emergency is too big for your emergency fund to fully cover. If that dipping in is low-likelihood and the amount is, e.g., the extra 18 months' worth of expenses you invested instead of keeping in your emergency fund, you're most likely to come out ahead.