Worth recalling, before we commit to the Withdrawal Agreement transition period, quite how extraordinary it is, in democratic terms, that the U.K. will be immediately and automatically be bound by law and decisions it which it and its voters will play no part.
See eg from the FT article quoted by Kirsty. 
The EEA is, really, nothing like this: narrower scope, governed by institutions with own-country representation, and far greater ability to refuse to go along with unacceptable EU measures.
For all these reasons, WA raises IMO a serious question about its compatibility with basic ECHR standards: see . IMO there is a serious risk of a successful Strasbourg challenge.
People have complained about the EU’s “democratic deficit”: but this is democratic bankruptcy.
And the idea that the WA period can sensibly last for only 20 months is for the birds - hence the need, as @FabianZuleeg and @tobiaslock have pointed out, for a provision allowing it to be extended.
But though that provision is necessary to avoid chaos and a cliff edge in 2020, it will simply perpetuate the democratic bankruptcy.
And NB that one indubitable advantage of moving to the EEA is that it would reduce - perhaps more or less eliminate - the need for such a democratically bankrupt arrangement. /ends
