The palace intrigue trope is the backbone of most sageuk and it is the focus of this show as well. Amnesia is another popular trope that is fairly timeless as far as plot device goes. Combine the two and we should have an engaging plot full of potentials, right? Well, sort of, but only if the writer-nim was up to the challenge. 
100 Days actually started well enough. The flashback was quite impactful but in their rush to set up the OTP's backstory and their fated love story, it gave away too much of the main characters' stories and motivations. With no change to the rationale over the intervening years, that removed a lot of the "intrigue" from the palace intrigues. This is not necessarily a lost cause because the plot switched up with the ML (a Crown Prince, no less) suffering an injury which brought on amnesia and he was a fish-out-of-water in a backwater village. This conveniently set up the fake marriage trope with the FL. Hey, when you already ticked two major trope boxes you might as well go for the trifecta!
The Show has a dash of romance, a hit of angst, a measure of intrigue and a sprinkle of comedy but nothing really dominated. It really felt like the Show is fishing for a genre but it swung from a sageuk to a rom-com then stretches of full-on melodrama and bits in between. None of the traits stuck though. This drama doesn't hit you with the feels but more like flick you with a wet hanky.
The writer-nim did try to make us ship the CP by using all the clichés in the playbook. In that sense, I have to give full credit to the FL (loved her in Suspicious Partners) as she did most of the heavy lifting here. The ML was handsome but he spent way too much time being stoic/poker faced/detached or feeling unease. That is a source of frustration at times as all you see is his stony countenance even though the script is clearing saying "you are in love" (is that the idol curse?). By the time he started to show some real emotions later in the show, the script dumped us straight into a long stretch of melodrama and he underwent a full face Botox treatment. This could have been a decent rom-com but the chemistry of the OTP was slow to develop and always felt a bit scripted which is a shame.
The Show wasn't bad and it was watchable but the writing has some issues. Old cliché power struggle plots were the only game in town and they were hardly original. Most characters were standard archetypes with little growth so an archvillain was just that. A petty local bully stayed petty. The found village/family setup was nice and sweet. The close knit set of villagers provided the thankless backbone of the show and some colour. Even the so-called machinations and scheming were fairly transparent and is oft given away by long expositions (even though the viewers are well aware of the plot by then). The odd revelations were more like confirmations and not surprise twists.
There was no doubt a HEA ending was mandated but the last couple of episodes were really awkwardly written (even by the Show's standard). After the longest set up regarding the ultimate showdown between the ML and the antagonist and how they were manoeuvring to outsmart each other. When it happened, it was a disappointing fizzer. It was poorly set up and poorly executed. Why did the ML charged into war still wearing his fancy court robes? There wasn't even any attempt to put on the simplest leather armour let alone anything befitting a royal personage. Speaking of THE war, it was more like a skirmish. It felt like an afterthought with minimal attention to details (did they run out of money?). To be honest, they could have skipped that completely and still achieve the same outcome. There was little narrative logic as the script rushed all the sub plot to conclusion and gifted everyone a happy ending. It was not so much "no questions asked" but more like "ask no questions and hear no lies".
I really felt sorry for the FL, she hasn't been in many dramas since Suspicious Partners and this is not a career highlight. She did her best but the drama let her down.
OST was serviceable but repetitive. Rewatch value is low. Not that many swoon worthy moments to relive.