TLDR: Its genuinely viable and very fun.

I genuinely believe this is how sylas is meant to be played. I get pretty grossed out when I see click bait Sylas videos with luden and first strike. This build allows for sylas to engage into the fight with a teamfight ult and stay there winning off the sustain in his kit and items. The buffing of tank stats across the board aswell as the buffing of frostfire, sylas W and E all spell great things for this build. Just give it a try.

TLDR: Frostfire procs fimbul making you sticky and tanky while demonic provides enough damage to carry fights in the mid/late game.

Fimbul on its own without frostfire doesnt make much sense. It would only be procced by the second half of your E and Q which I believe isnt consistent enough to justify the purchase. Thats why we go frostfire first to allow frostfire passive to proc on our autos and subsequently proc fimbul passive. Allowing you to be super sticky and take even less damage while dealing a ton with demonic and immolate. Demonic Embrace really brings the entire build together with its health and AP rounding out our damage and sustain wonderfully. BUT a big caveat of this build it relies on you not running it down two dumb deaths can really set you behind and be painful as you lose alot more when your screen is grey than if you were building standard AP. You lose precious tear stacking time and dark seal stacks which you kind of rely on accruing to have a bit of AP to last you till demonic.

TLDR: More consistent throughout the game and allows you to be that frontline for your team while dealing consistent damage.

I think the only advantage standard AP has over this build is the mid game. There can be a lull between fimbul and demonic if you dont have dark seal stacks, or unluckily have alot left to stack on your winters approach. But where this build shines is the consistency within its game plan the late game isnt nearly as bad as standard AP and I would 100% of the time rather have bamis ionian than lost chapter ionian. You are always tanky with consistent damage what you lack in burst you get back 10x.