ugly.swift

// Option 1: if..elseif + optional unwrapping conditions

private func handleNotificationAction(id: String?, userInfo: [NSObject: AnyObject], responseInfo: [NSObject: AnyObject]?, completion: () -> Void) {
    let taskId = userInfo["extra"]?["task_hash"] as? String
    let projectId = userInfo["extra"]?["project_hash"] as? String
    let comment = responseInfo?["UIUserNotificationActionResponseTypedTextKey"] as? String
    
    if id == "comment", let task = taskId, comment = comment where !comment.isEmpty {
        
    } else if id == "invitation_accept", let project = projectId {
        
    } else if id == "invitation_decline", let project = projectId {
        
    } else if id == "star", let task = taskId {
        
    } else if id == "complete", let task = taskId {
        
    } else if id == "show" || id == "comment" || id == "delegate", let task = taskId {
        
    } else {
        // assertionFailure
    }
}

// Option 2: switch + force unwrap

private func handleNotificationAction(id: String?, userInfo: [NSObject: AnyObject], responseInfo: [NSObject: AnyObject]?, completion: () -> Void) {
    let taskId = userInfo["extra"]?["task_hash"] as? String
    let projectId = userInfo["extra"]?["project_hash"] as? String
    let comment = responseInfo?["UIUserNotificationActionResponseTypedTextKey"] as? String
    
    switch id {
    case "comment"? where taskId != nil && comment != nil && !comment.isEmpty:
        
    case "invitation_accept"? where projectId != nil:
        
    case "invitation_decline"? where projectId != nil:
        
    case "star"? where taskId != nil:
        
    case "complete"? where taskId != nil:
        
    case "show"?, "comment"?, "delegate"? where taskId != nil:
        
    default:
        // assertionFailure
    }
}

How about going completely left field and doing something like the following:

typealias NotifyActionSuggestionHandler = (taskId : String?, projectId : String?, comment: String?, completion: () -> Void) -> ()

func handleComment(taskId taskId: String?, projectId : String?, comment: String?, completion: () -> Void) {
    guard let tid = taskId, let c = comment where !c.isEmpty else {
            print("Comment Data Error")
            return
    }
    print("Comment: task(\(tid))")
    completion()
}

func handleInvitationAccept(taskId taskId: String?, projectId : String?, comment: String?, completion: () -> Void) {
    print("Invitation Accept")
    completion()
}

var handleStar : NotifyActionSuggestionHandler = {(tid,pid,com,comp) in
    print("Handle Star")
    comp()
}

let commands = ["comment" : handleComment, "invitation_accept" : handleInvitationAccept, "star" : handleStar]


func handleNotificationActionSuggestion(id: String?, userInfo: [NSObject: AnyObject], responseInfo: [NSObject: AnyObject]?, completion: () -> Void) {
    guard let actionId = id, let action = commands[actionId] else {return} // Or assertion failure (fail fast)
    action(taskId: userInfo["extra"]?["task_hash"] as? String, projectId: userInfo["extra"]?["project_hash"] as? String, comment: responseInfo?["UIUserNotificationActionResponseTypedTextKey"] as? String, completion: completion)
}

let user : [NSObject:AnyObject] = [:]
let response : [NSObject:AnyObject] = [:]

handleNotificationActionSuggestion("comment", userInfo: user, responseInfo: response, completion: {print("Comment Done")})
handleNotificationActionSuggestion("invitation_accept", userInfo: user, responseInfo: response, completion: {print("Invitation Done")})
handleNotificationActionSuggestion("star", userInfo: user, responseInfo: response, completion: {print("Star Done")})

Keep functions small, easily extendible and avoids complex conditionals/switch on strings - also brings testing benefits. (Although I do like narfdotpl suggestion)