I'm silver 3 and my current "main" is Omen but i'm not very good with his TP or his ult. So i wanted to switch to Brimstone but i never see him in high elo games so i don't know if he is viable ?

As a Cypher main with more than 1.2k hours on Cypher, he has not been able to keep up with the other sentinels. His ultimate is ineffective in a lot of situations and depends on a downed enemy. Cypher and Killjoy are much better alternative...