Even I can see the appeal. Your cousin, your best friend, your quirky aunt – they've offered to officiate your wedding. It's free! It's personal! It's... probably a bad idea. Here's why.
They'll be decidedly nervous. You want the person presiding over this critical moment to project calm and confidence, and instill them both in you. Inexperienced or first-time officiants will likely bring a nervous, hesitant energy to the ceremony - and you'll feel it too. They need to mitigate your anxiety, not the other way around.
Just because they aren't afraid of public speaking doesn't mean they're good at it. Expert orators are the ones who study their craft, and engage in that craft a lot. One's willingness to speak to their peers doesn't guarantee the message will be appropriate for the occasion, or delivered with grace, nuance, and aplomb.
They rely too heavily on cliche or sarcasm, or think they're a comedian. The first two are bad enough, but the hissing silence of un-laughed at jokes can really harsh the vibe. You'll feel the cringe.
Sure, they know you really well - but that's a knowledge base, not a skill. Without the real-world experience to turn it into a great ceremony, no matter how intimately they know you, they're likely going to miss the mark.
They make rookie mistakes. They forget key components of the ceremony, or do things in the wrong order, or make clunky transitions. Their cell phones make noise. They forget to have guests stand for the bride, or forget to have them sit down, which ruins their ceremony photos. These small mistakes can have a dramatic impact.
They can't problem-solve on the fly. When something at a ceremony goes awry, the whole room - but especially the two of you - will naturally look to the officiant for a solution. Your friend or family member probably won't know how best to recover if there's a slipup.
They bail constantly. It's often a work-related excuse, but others claim personal problems or busy schedules. Some straight up cop to biting off more than they can chew. When it happens, it rarely leaves you with as much time as you'd like to find a replacement.
Bias, I Cry-us!
I'll cheerfully admit my bias in offering you this advice. Of course the professional wedding officiant is urging you to hire a professional wedding officiant, and not the free alternative! I still hope you'll carefully consider it, and give it the most forgiving interpretation - that it comes from my genuine desire to see everyone enjoy a stress-free and memorable wedding day.