There might exist some other fingerprints that can lead to the knowledge of what browser you're using (the existence or non-existence of some feature, etc). If your User-Agent contradicts those others fingerprints, you become very easy to identify. It's really difficult to consistently pretend having a given configuration unless you are, well, actually running this configuration.
More generally, the more you use technologies to protect your privacy, the more you increase your fingerprint entropy, that is, the more it's easy to identify you.[1]
I wouldn't advice those "spoofing" tools, but rather to use the browsers' settings at the maximum of their capabilities (disable third-part cookies, delete cookies when closing the session, set Flash to 'Ask to activate', etc) and just the minimum extensions set to block third-part requests in a first-place (Disconnect, etc).
An offline analogy might be "I didn't want to stand out in a crowd, so I had a friend carefully embroider a custom mask that was thoroughly different in every respect from my normal appearance, and I started wearing randomly-chosen items of clothing bought at thrift shops every day".
Tor Browser Bundle is working on this: "Design Goal: All Tor Browser users MUST provide websites with an identical user agent and HTTP header set for a given request type. We omit the Firefox minor revision, and report a popular Windows platform"
I understand that spoofing one's user-agent can be very effective at fooling malware designed to for specific browsers. However, in practice I have ran into strange problems when doing this. Is there a generic user agent string I could use that would hide my browser vendor and make all sites display content designed for evergreen browsers?
https://addons.mozilla.org/en-US/firefox/addon/random-agent-...
because of this:
https://www.privacytools.io/#browser