His opinion was that I could tie an overhand for the master point and
be just as safe–the only downside being that it would be more
difficult to untie after being loaded.
Yes, this is correct.
Many people seem to think that commonly used climbing knots can slip if there's not enough friction to make them hold, and that we should choose a knot based on whether it's "stronger" against this type of failure. Actually, slipping is not a mode of failure for any of the types of hard knots customarily used in climbing (as opposed to something like a Munter, which is designed to slip, or a granny, which is not used in climbing because it's not secure).
The only mode of failure for an overhand or a figure-eight is that you can put so much tension on it that the rope breaks. It simply doesn't happen in any realistic climbing situation that we put so much tension on a climbing rope or a cordelette that the cord breaks, and therefore this is not a realistic concern for these knots. (Tests do show that one type of knot may cause the rope to break under less tension than another. This is because the rope tends to break at a place where it's tightly curved. However, the amount of tension required is still just not something that happens in a climbing application.)
As Shem Seger pointed out in a comment, the application referred to in the question is also one in which we have multiple strands. For example, if the anchor being described in the question is being built with a cordelette made out of 6 mm nylon, then we have 6 strands coming down from the gear to the knot, and 3 independent loops forming the master point.
It's true that a figure 8 is easier to untie than an overhand, and that's why we use it to tie in on our harness. However, that's not a particularly realistic concern for the application described by the OP. The question describes a triply redundant anchor, which points down. The only direction of pull it is likely to experience is down, i.e., you're going to use it to support body weight and possibly catch a top-rope fall. Those aren't likely to produce enough tension to create any serious problems with untying the knot.
A lead fall (unless it's a factor-2 fall) is going to generate an upward force. It could be a much bigger force, but because it's upward it's actually not likely to stress this knot at all. The concern here is not that the cordelette will break, it's that the anchor probably isn't designed to hold against that direction of pull, so you may have pieces popping out. This is one of the reasons why it can be a good idea to supplement your anchor with a piece that is intended to hold against an upward pull.
In the case of a factor-2 fall, the big concern is that this can be an extremely violent fall that can actually destroy your pro or break the rock. The cordelette is not the weak point in this situation.
A more realistic consideration here is simply how much cord you have. A figure 8 uses up more of your cordelette. If you don't have enough length available in the cordelette to tie a figure 8, then that would be a good reason to use an overhand.