They're a lot of fun actually. Especially if you like open world exploration. They add a lot of very cool maps.

Most of the game is solo-friendly. Some things are very difficult to solo, but if you get really good at the combat system it's still possible.

Coordinated end game party stuff isn't my bag so much, so I'm not sure about soloing it. There are a lot of different things you can do with different names that all amount to dungeon/raid style content. The things actually called "dungeons" are all from the original game and are really old at this point. The newer 5 man dungeons are called "Fractals", and there are also 10 man raids, and 10 man boss only encounters called "Strike Missions" (basically a single raid boss with no dungeon around it).

Heard something about where you can do dungeons solo-party? I think that was gw2.

"Dungeons" as a general term is understood to be "group content".

In GW2, these are:

• Dungeons, which you go through with the Core (base game) story, then are available as a max level (don't worry, you get there fast) content with 3/4 different paths each. They are made so you go through with 5 people. There's a LFG system which (while ugly,) works and if you don't find one you can just put up a group yourself with the title "Ascalon Catacombs - Story Mode" or "Caudecus Manor - 1st Path (or P1)", and it should fill up pretty fast. However, once you have enough mastery over your character, you should be able to solo them. I don't recommend expecting to be able to solo them easily right off the bat, however.
• Fractals, which are also 5-men content, and are accessible from max level. Those go in difficulty degrees from 1 to 100, and also have a LFG section dedicated to them. Like Dungeons, you could I guess solo them, however this gets harder the higher level you go, and I honestly consider people who can solo the higher levels as madmen.
• Strikes are 10-men content, and are shorter (one boss) events, and go from easy to hard content to go through. They are usually considered end-game content already. With 10 people. I suppose some of the easier ones are soloable, but that seems like an absolute slog to me and very un-fun. A few of them have challenge modes for the challenge-addicted.
• Raids are older content (though they are bringing back a new one in the current expac) which are the highest level of difficulty available of content (though the challenge strikes are harder than some raids, iirc). Those are harder to get into because of the smaller population of dedicated players, you usually need to find a raiding guild to get some training runs in so you can then find a regular roster. I'm honestly not sure how it is nowadays, but my experience with them is limited because I did not enjoy that difficulty level.
• Group Instances are 50-men content, limited in number but generally accessible in difficulty. You just generally pop up at the entry spot whenever it is time for the public entry, then get randomly assigned to an instance completing it (although you can find specialized squads doing them if you look). Those are not soloable, but you don't need to speak with anyone as people generally know what they have to do and manage to get through.
• I guess World Bosses count also? Those are open world, so just be there a bit before the event starts and you'll have enough people waiting to do them on the map, so while again they are not "solo", you don't actually need to talk to anyone to get your fill of content.