These two categories of costs seem pretty straightforward. And they are, or at least until one entity takes on multiple roles. For example, a big institutional 401(k) provider might be the custodian, the third-party administrator, the advisor, the record keeper and the investment manager. For all this, they might say, "We'll drop out our asset-based fees, and you'll just see a single, low expense." On the outside this looks great. You've bundled and saved. But, what about the funds that big institution is selected for the plan? What if a portion those fund fees got kicked back to said institution? If that were happening, the institution might be incentivized to put you in funds with high kick-backs instead of suggesting the most optimal funds. In this case, sure, you're fixed expenses are low, but the bulk of your costs will grow as the plan grows. We see this sort of situation all the time.
Another issue we see, with insurance companies most commonly, is administrative fees which are tied to the total number of assets within the plan. Instead of fixing these costs, they, once again, grow as the plan grows. Worse yet, it's unusual to see the fee schedule taper over time. Fee structures such as these might make sense with really small plans, but it usually isn't long before this arrangement doesn't suit plan participants.