By the time you pay for each little piece with Allegiant, plus added transfer to/from Sanford, it's often not cheaper.
PPs have covered most of the issues (you get "nickel and dimed" for everything, so you can't glance at the price and think you are getting a great deal because there will be many additional costs, and there might only be a few flights a week out of your airport).
But maybe this wasn't stressed enough.
Allegiant flies into Sanford airport, not Orlando International Airport (MCO). Some people like Sanford because it's not as busy and hectic as MCO. But it is considerably further from Disney World than MCO. If you google it:
Orlando Airport (MCO) is 16.6 - 21.2 miles away from Walt Disney World Resort depending on your route.
Sanford Airport is about 40 - 50 miles from Walt Disney World.
If you're going to rent a car anyway, then maybe you don't care. But if you are use other transportation, you have to factor in the added cost when you are pricing out your trip.
As an example:
Tiffany Towncar round trip between MCO and a Disney hotel: $165
Tiffany Towncar round trip between Sanford and a Disney hotel: $260
If you plan to use Uber or Lyft, getting to your hotel from Sanford could cost twice what it would to get there from MCO.