Whenever I play the game I kinda just default into always equipping the artifacts and relics that fit the characters but are they even actually good? Compared to high level gear it often seems like a waste to actually use them. What are the good artifacts / relics and do you need specific builds that make some of the not so good ones great?

There are good and bad relics. You can google list of them with stats and decide for yourself. Choice of artifacts and relics is dependant on what kind of party composition you have.

You can only get 12 (or 13 not sure) artifacts in one playthrough. After you got 12 (or 13) you will stop getting them, so sometimes it is wise to reload if you get unwanted artifact/relic.

thing is that on weapons for example wouldn't you just rather have elemental enchantments early game and swift + vamp later on? wallace / terminus seems very good but that's about it from the weapons I've seen. Splitter and thor always seemed interesting but I've pretty much always ignored axes so I don't know if they can / are actually good anywhere.

You must be more specific about what MM you are talking about. Different weapons and weapon skills are good for different classes in different MMs.

talking about 6/7/8, hasn't it always been all about spears, daggers, maces and swords though?

MM7 introduced GMing skills but it made restrictions on how far you can master weapons depending on class.

MM8 switched some things around and totally changed class system.

And, of course, each of those games has its own set of artifacts and relics with totally different stats, so need to look per game anyway.

yes massive differences there, in mm6 everyone was using spear / dagger, in mm7 everyone was using spear / dagger / mace, in mm8 everyone was using spear / dagger / mace with the occasional sword in each of them. sure the artifacts and relics are somewhat different but the list of actual good ones is quite short.

Treat them like any other piece of equipment. If they are the best stuff you have available, then use them. If not, then don't.

Yoruba is useful platemail for all the immunities that it provides.
Old Nick is a pathetic dagger that gets tossed in a chest and never looked at again.
Mash is used to pass Strength challenges, otherwise it is shelved because of the stat reductions.

In MMVII, it is easy to counteract the downside in Artifacts; in MMVI, not so much. In MMVIII, Artifacts are just one more lopsided event.