Potential merits: None for the dog. For the human, MAYBE the reassurance that you're "still a man" 'cause your dog has a nice big sack of (fake) balls hanging there for the world to see.
Potential downsides: Expensive, pointless, cosmetic only, complications of surgery to install them (I DO NOT HAVE EVEN A SINGLE LONELY BUNNY-TURD WORTH OF A SHIT TO GIVE about what their advertising literature says on the topic of how safe they are, so don't bother to mention it) the fact remains, no matter what their claims, that when you cause a wound, for *ANY* reason, be it good, bad, or in between, the potential for bad results - infections, including potentially fatal ones, failure of the wound to close/heal (relatively rare, but does happen) scarring, etc - is always present, and quite real.
Lemme save you a "yabbut": I couldn't g\ive a rat's behind what their sales literature says. They'll say ANYTHING they can get away with to convince you to give them your money for this pathetic, purely cosmetic, and aimed only at making the human feel better GARBAGE.
You asked for an opinion, so I'll give you a straight, all-the-bark-still-on, no punches pulled, totally honest, no-nonsense and no apoligies for it answer: Neuticles exist for no reason other than to separate pathetic, insecure posers from their money as they try to convince themselves that they truly are "MANLY MEN!" because their dog who had its nuts cut off (for whatever reason, good, bad, or otherwise) looks like it still has balls. That's it. That's all. If I had my way, the company would be banned from doing business, and anybody who actually fell for their would be given a one-way ride to the gulags because they're too stupid to be allowed amongst sentient humans.