But nowadays the signature campaign is becoming strict to the requirement of enrollment. Some of them don't allow to farm their signiture campaign.
As signature campaign managers don't have access to IP, it is easy to join from multiple accounts. If you post in same style, they might catch you.
And It is difficult to manage to make all the quality posts with so many accounts. I think it depents on how long of your spare time you have got to post them, but not spamming them otherwise you get banned. You should figure out the balance yourself.
You are right. Some people still post in a topic and make replies to their posts with their alts. It is still hard but many of them go unseen. Recently, some of the top posters were banned(Madness, marcotheminer etc...). Mods have been more strict and they are doing their job pretty well. IMHO you can be in safe zone if you only have one account and is making on-topic post. To make on-topic posts, SaltySpitoon gave a great advise:
Keep in mind, the only real way to avoid spamming is to think critically about posts and topics you respond to.
-MZ