I think a big part of the reason why self-taught singers struggle with singing, is because they don't understand the process of how to learn to sing. In fact, I imagine this also extends to people who also have a vocal teacher, but it depends on the teacher. In any case, once you understand this process you can gauge whether your vocal teacher is good or not.

It's not that the process is difficult, it's just people don't know about it, and so they make up all sorts of other theories which simply don't make sense. It's also why people feel like they don't know if what they're doing works, because they literally don't know. And that's a problem.

The process is essentially this:

1. Make a noise i.e. sing.
2. Gauge that noise using some kind of physical or mental measure.
3. Analyse the result. Change Action.

What most people do is step 1, and then either skip to step 3, or their learning ends there. Primarily, they don't know how to gauge if what they're doing is correct or not, and if you can't do that, you're basically guessing. I imagine this is how a lot of self-taught singers feel: a bit lost with what they're doing.

Now there's nothing wrong with guessing per-say, it's just that it makes what you're doing very difficult to reproduce. Especially if you're singing the next day, and you've completely forgotten the feeling of that technique.

Now what bugs me is when people say "well, that's why you need a vocal teacher to tell you" but I don't think that's an adequate response. Yes, a vocal teacher can be helpful as your ears to guide you, but ONLY if that vocal teacher also teaches you how you can gauge it yourself. But given that, online resources should be enough if they also teach you how to gauge yourself.

But then also, a huge part of the issue with online resources is that a lot of them don't teach you to gauge what you're doing. So they give you an exercise, and they expect you to understand it based on how they're replicating it to you. And that's it. Also, simply explaining the dynamics of how it works isn't enough, because it's merely describing a feeling. Not how to check that feeling.

Here's an example of a good learning process.

1. Sing a vowel, going up and down in range.
2. Put your thumb underneath your jaw, and gauge whether it's moving up or down or not.
3. Adjust your singing until you can sing without tension/movement in your jaw.

So in essence, if you're learning online, ensure that the resource teaches you to gauge what you're doing. If not, then it won't be very effective.