Statistically, they aren't bad matchups for LB, and she actually wins slightly more vs them than normal.

Sylas doesn't have kill pressure on you if you don't W on top of him and get baited by his healing, since his CC is telegraphed by the dash. He is also one of the few champs with worse waveclear and push than lb (pyke is the other one), so he can pretty much be ignored while you roam.

I feel like other champs like Akali, are screwed by Sylas and Viego WAY more than LB. LB's ult is kind of shitty for sylas, esp if you cheese it and give him sigil, and she is ranged with a dash, so she can farm pretty safely compared to most squishy champs like her.

For both of them, I like to stack early manaflow band for early sustain, since they are easy to hit with Q, rather than take inspiration and try to force a kill before 6 like normal.