I agree silence is a fantastic ability, but depending on what class/builds your opponent is doing it may not be worth upgrading past the first level. If my opponents are playing speed reg, but i have an UB on my team, id take a more support oriented sedna and probably pick up the minion buff skills. On the other hand if you are playing against a rook, silence/pounce can absolutely devastate.
Sedna can be very hard to displace, you can hold your own against a couple of demigods at a time if you are conservative, which can buy your team valuable time to reinforce. Late game Sedna with the bracelet of rage is total destruction as far as the eye can see.