If the inside is open and clear it's a no brainer easier too, but all those structures on the inside take a long time to get out and then restore back, including fairing work which is very time consuming.
If one is already there doing everything else, fine. Way easier.
If the pieces of ply or board material are all cut by layer and prefit and the first is bedded with plenty of thickened resin on the trowel plus 1.5 oz mat loaded up, pulled in via prefit screws and blocks from inside, nothing is separating that joint. The key is to prefit and cut everything beforehand incl. how to pul first layers in tight from inside then remaining layers from outside in. My transom is 2.5 inches thick, several layers would go back with joints in each layer staggered. I'd cut the outer skin futher away from edge as I've seen, more time for me but more skin to scarf (more cost to customer on pro jobs).
Then with slow hardener out of the sun and mod temps, it all goes in as one shot no stop work.
Come back later for outer skin.
I'd add more glass to inner skin first, all by working in a sitting or standing position, no knee pads, no climbing in and out of boat. When it comes time to match up outer skin scarfed back, one can 50 grit belt sand off a ply of wood easy since it is ply by ply - if new transom is too thick, or add glass if more is needed.
I would say most transoms are done from the outside other than for full restorations, the last few boats I've seen here started off as gas tank and soft floor spot jobs turned out to be replacing 90% of the wood in boat, and a full season or more to restoration.
RAYBO would be a good source for recommendations on any boat model and age, and the quote that goes along with each.