After years of catering for my own parties (I'm a keen cook) and helping friends do the same I think a lot of the problem is that people perceive that buffets are the less expensive option.
That food in bulk has the ability to magically stretch further as if Jesus is on hand with his 5 fishes and a bread loaf feeding the five thousand - it really doesn't and you'll be needing a miracle if you think 50 scones will feed 75 guests on the basis not everyone will want one (maybe, but some people will have two or three).
The issue is you have no opportunity to potion control, so realistically a good buffet will be more expensive than a simple portioned meal because you need to over cater.
It's perfectly possible to do a great buffet but you've got to follow some key rules.
You can't assume everyone will eat (or rather plate up) sensibly. Catering for 100 people and providing only 100 jacket potato's isn't going to cut it. It doesn't matter that it seems reasonable - far more people than you think will stack their plate (even if they don't eat) twice/three times the amount of food than even the most generous portion you can imagine.
Children don't plate up less - they aren't going to be cutting items up to make smaller portions. They'll just leave half uneaten. So unless you have child specific food you need to count them as an adult portion (if you do have child specific food also don't assume adults won't scoff some of it).
Don't assume that only vegetarians will eat the veggie options and thus that you need less of this than meat based dishes. You'll actually need to assume everyone will eat vegetarian dishes and only 95% of people will eat meat. Ideally a good buffet will have multiple veggie options in large quantities.
Cutting items into tiny portions won't make it go further - you might be able to cut a quiche into 12 wafer thin slices but that won't mean it will feed 12 people - often the reverse is true, because people are more tempted to take three slices and not "feel" greedy rather than just one reasonably sized slice.
Don't put all the food out in one go. This is especially true if you have hot options - congealing hot food isn't attractive, neither is plates of food that's obviously been picked over by 30 or more other people previously.
You need to keep replenishing the buffet so the food remains at its best and the presentation is as attractive for the person last in line as the person who was first. This means you/waiting staff being on hand to do this.
If guests have specific dietary needs then plate theirs up ahead of time and keep separate - some dick will snaffale the gluten free bread rolls just to try them.
All the reasons above are why buffets should be more expensive than a portioned option - when they are not is because corners are being cut and you'll definitely notice it.