I am a huge proponent of learning the minimum amount of good knots to deal with the situations you need, and not a single one extra. For my self, a san diego jam covers direct tie with any line type, Alberto for leader connections, and a uni-snell (but I am getting away from snelling). I agree with others for the most part by saying, whenever possible, tie direct with one of those types of knots.
There are a few cases where I don't mind a small barrel swivel. Fluke setups, and drop shotting. Yes, I said drop shotting. And I am sure there are a few other justified situations that reasonably call for a barrel swivel that I didn't mention above, but I digress.
I am on Erie every weekend, so naturally I drop shot a ton, so let's address the issues:
Line Twist: Despite regurgitated non-facts, line twist is a verrrry real thing with braid, especially with certain techniques (like drop shot). Yes, I like a good connection knot, and yes, I try to hook my bait as straight as possible, and yes, I reel in slowly. But, if you drop shot frequently, line twist can be unavoidable, this is where I like a barrel swivel for drop shotting.
Adding Weight: Just like in a fluke setup, a barrel swivel can be the answer of solving the above problem, while adding a little up-the-line weight to get a certain desired action out of the fluke or weightless plastic. Yes, you can use a pegged bullet weight or split-shot, but a small barrel swivel is very stream-lined and gets the job done well.
Knot Fear: While I would never encourage someone to tie unnecessary additional knots, this is the most common negative that people will bring up with this or similar topics where you add an element to your presentation via more knots.
So I'd like to say this: whether you have one knot or 5 knots tied between you and your bait, if you are breaking your line/knot, the number of knots is not the issue.
Choice of knot: Learn a high strength, yet simple knot to tie. Many people get attached to certain knots "because that's what they've always tied". Do your research, do your own testing at home. If you are using a poorly designed knot, you are failing before you get started. People like to also learn unnecessarily complicated knots just for a debatably negligible strength gain.
Knot Quality: Of course, no matter then knot, if tied improperly, can fail. This is why I am a big believer in as simple as possible. If the knot I tie is easy, simple, hard to tie incorrectly, and it has a a good average break strength, I will absolutely not use a significantly more complicated knot just to achieve 95% or even 100% breaking strength when I already had 90%.
Equipment Settings: If you setup your equipment improperly relative to your line/hook, you will fail. Rod power (even action) and drag are the keys here. You can tie the best knot in the world that has 200% strength (kidding) with 4lb line, but if you are frog fishing in lilly pads, using your xxxheavy rod, with a locked-down drag, you will fail, and it's not the knot's fault.
In summary, of course every additional knot gives you an additional chance to mess up tying a knot, but if you do your due diligence and ensure each knot is a good designed knot, properly tied, and have your equipment set correctly, you should be fine if you want use an extra knot.
Maybe i'll make a video this year with a bunch of daisy chained leaders/barrel swivels and catch some fat smallies with the same reel/rod settings as if I was only using one direct tied knot!