Not sure how unpopular this actually is but I've seen a lot of posts attempting to discredit Sylvain so I feel like posting this. I'm under the impression right now that Sylvain is one of the top combat units in the game, only really losing out to early game Felix and raging storm Edelgard. He has huge damage stacking potential from supports with +3 from both Ingrid and Felix. Hilda standing adjacent to him not only activates his skill but also her personal getting him +5 more damage. To put his damage stacking into perspective at base wyvernlord he can OHKO Ladislava with ruined sky and 2HKO Edelgard in ch17(Easier with Paladin) with swiftstrikes both on maddening (The setup for Edelgard is admittedly a little impractical but it's mostly to show how high his damage numbers can go). Only a few units have 2 damage boosting supports, the others being Felix, Ingrid, The Lords, Byleth, and Annette.
His defenses are super good. He can obviously get +2 from his personal but his big selling point is battalion vantage. Silveraxe+ on a Wyvernlord can usually OHKO magic units and often OHKO bow units as well even on maddening. This means with retribution he's completely immune to the most damaging units against a wyvernlord. Obviously he can dismount against archers but the mage problem is huge for wyvern tanking since magic ignores terrain avoid and Sylvain just doesn't care. Battalion Vantage is also rare on other units being learned only by Edelgard, Felix, Lorenz, Catherine, Dimitri(A rank), and Ignatz (A rank).
I've seen many people complain about hit rates late game and use this as an argument to claim Ferdinand is better than him. I'm not here to discredit Ferdinand as he's also a really good unit but hit rate problems was something I also had trouble with when I first started. Accuracy rings, +15 hit battalions, A support adjutant's and linked attacks, prowess skills, and even rally dex can also boost your hit rates into incredibly high numbers. Lance of ruin does have a fairly weak hit rate but when facing a dodgy opponent the player can always pull out Luin for it's 90 hit(Or a Silverlance+ if they don't have that).
In short Sylvain has strong offensive options in his damage stacking. Ignores AS with swift strikes. Has great defenses with vantage and can pull great hit rates. The only thing that really competes with him is Edelgard on CF route due to managing to make a combat art stronger than swift strikes, and early game Felix due to having good enough combat to ORKO even on maddening.