Hey - I was researching this last night; for background, the following thread seems to have hashed out (or at least identified) some of the issues fairly well:
https://forum.atlas-games.com/t/suggestion-for-crafting-mage-needed/9026/1
The main benefit seems to be the "if it doesn't take an EF of 9 for Creo, you don't need to roll it." That's it. However, If you want to munge some of the old (core) rules with the newer (A&A and HoH:S) rules, the following may work (Note that all of these are more-or-less subjective interpretations on my part, from the above thread).
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Originally, Creo only needed a finesse roll to make it look nice; regardless of your casting, you'd still get something functional. In contrast, with Rego (if you failed), you'd get a pile of junk. (Although this may have been superceeded by later rulings.)
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Creo might be able to use "complexity magnitudes" to make the EF easier. (This is a holdover from 4th edition, most likely) - it looks to be +3 EF per magnitude. So, an EF 15 Creo spell, with +2 mags for complexity, can get down to +9; and thus you don't need to roll at all. This seems to be what Conjuring the Mystic Tower did.
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If the Creo spell gives you an exact replica of a given item, you don't need to roll Finesse at all. (suggested by a few of the Creo effects, but may simply be an oversight). There are some examples in Changing Mythic Europe that don't have an EF described, but the Creo spell creates an exact replica of a specific ship, for example.
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Both Rego and Creo pull from the Realm of Forms to make the magic easier; that is, you visualize the ideal X, and then use the magic to build X. So potentially, if you're using either to partially build something, you will need to know how it works via an ability roll. That is, if you want to build a Cathedral in pieces, you need to know how the Perfect Cathedral Wall holds up the Perfect Cathedral Roof. If you did it all at once, you wouldn't have to.
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It's not QUITE clear that Creo needs to use the time factor when determining the difficulty. (Although it probably does.)
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It's also not clear how many workers you get when calculating the time it takes to build something with Rego. It looks like, by default, you get 1. Thus, you may need to put additional orders of magnitude to actually get enough workers to build a castle, as 1 person can't do it by themselves. (Although this also isn't clear, at least to me - increased magnitudes for size MAY increase the functional number of workers.)
Anyway - those were the issues I gleaned from that thread.