On paper, cyphers kit is really strong for defense and map control in general. But to get straight to the point, it's really what the other agents have that make cypher really bad. Currently, 11 out of the 19 agents can get around or break cyphers utility without putting themselves in any danger. Not to mention, when you compare cyphers kit to other sentinels he does seem quite underpowered. Sage can easily split up or shut down a push with her slows, Chamber ... its chamber, and Killjoy has really good stalling capabilities with her nanoswarms. So, not only are there better options than cypher, but when you do go cypher, more than half the time the enemy team will have something to get around your kit.

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TL;DR -> Cypher is bad because everyone else is better + over half the agents can counter him