New(ish) player still, are uniques really that useless?

I have looked at guides, watched guides, and it all seems to point to that rares are the only way to go in order to get enough DPS for later end game content.

I have always liked the idea of finding some weird unique(s) in these types of games and making a build around it, instead of relying on so called meta.

Is that possible in POE?

Generally rares have more in terms of generic stats, means if you look for raw life, res or in case of weapons, weapon damage then rares are usually the way to go, especially on the top end. That said unique items often have special stats that rares can't get and in some cases, those stats make them worthwhile. However, even with that it's not an exaggeration to say that 90%+ of the uniques are pretty useless, in many cases because they are low level items, but even most high level uniques aren't worth it.

So to answer your question, it's possible to make off meta builds, it's also plenty possible to make creative builds using uniques BUT it requires a thorough understanding of the game and in most cases you'll only end with a mediocre result compared to just bruteforcing it with raw stats.

There are still good builds that rely mostly on uniques. For example see my character Voxx (Poison SRs Jugg). I completed 40/40 challenges with it.

Mid tier unique items like The Annihilating Light, The Covenant, Loreweave, Paradoxica, Dialla's Malefaction, and many others are still very popular. Also there are the high end uniques, which can be used in many builds, but very expensive (Mageblood, Headhunter, the new flasks, Nimis, Original Sin, etc).

Rares are generally much greater overall power but some uniques take certain properties to enough of an extreme that if you only care about that one aspect, you can't beat it with a rare.

Some (a minority) uniques CAN be best in slot depending on the build. But you can't just randomly slap on cool uniques and expect to have a strong character.

Examples of uniques that can be best in slot:

1) If you play spell totems, you cannot beat a Soul Mantle chest piece for damage, especially since they can come double-corrupted.

2) My Glacial Hammer Slayer this league uses a double-corrupted Frostbreath unique mace. You cannot beat that for damage until you go for something REALLY expensive as a rare. Since maces are not something the mega-rich crafters will make, you would need to make it yourself (think several thousand divines spent to craft) whereas I bought mine for peanuts.

3) If you are playing any kind of attack-based champion, you cannot beat a Perseverance belt with a rare.

There are many more of course.

it's 100% possible. guides just tell you not equip every uniq you find and wear only these, that feat your build idea well

They have been massively buffing countless uniques in the past year or so, and since 3.19 I believe that they had another round of buffing those uniques.

I dare say: it wasn't before, but now it is possible to equip full (or 95% full) unique and beat most* endgame content.

* most does not mean all

A lot of uniques have stats that are impossible to get in rares, but that obviously only matters if your build actually needs those specifig stats.

So rares are most of the time better for generic stats, but specifig uniques are often better for specifig build and there are multitude of builds that are only possibly because of some unique or combination of uniques.

Unique items are generally for early and mid-game till you get more currency, they're generally cheap and useful but
You can beat all endgame content with full unique items.
It's pretty easy. But the thing is when you complete unique items with rares you'll get much more profit dps wise, so it's better to use both in order to make the game easier and more entertaining.

Uniques are only used in certain cases, for example if your build relies on them for one reason or another. The Goldrim cap is a commonly used levelling item as it raises all elemental resistances by a significant amount. Another example is the Tabula Rasa which is a cheap way to get a white socket 6-link for your character so you can fully exploit the use of different coloured gems. Otherwise rares are the way to go.

Many uniques have modifiers that cannot roll on rare items. It enables the possibility of interactions that you could not have otherwise. There is also the phrase "build enabling unique" for a reason, so I would say no to your question.