Meta builds = less party efficiency (because people will have the same Classes & same buffs with a larger probability)
Meta builds in general are only self-sustainability and self-impowerment.
Party play in Re:build lives from bringing many different Classes into a party to have the maximum effectiveness.
Of course, these many different Classes need to have some sort of group support, otherwise they’re useless for party play (like e.g. all Archers and the majority of Scouts and Wizards and Swordsmen).
Sadly, there is no longer any room for cross Class-tree synergy since most mechanics which can be used/abused are cracked down upon with Re:build, leaving mostly in-Class synergy,
e.g. Icewall can now only be attacked by the casting Wizard and with Bokor going to Wizard, no Cleric can benefit from the Necromancer debuffs on summon damage.
This means in general that it has boiled down to playing a build one likes, if you want more efficiency you have to make a pure Class-tree party (e.g. only Clerics or only Wizards or only Swordsmen) to abuse skill-related synergies and to bring mutual benefits to the party.
For solo-play, all you need is one Class that deals high damage onto a large area, the rest is basically free and only matters for specific situations.
The foremost matter is dedication, you need a good weapon (Velcoffer) and a decent investment into skill enhancement attributes to be able to have a good output. You also need to make an effort to combine your skills in a way you have the most benefits from them.
As there are only so many skills which can be used effectively within a certain time period, a lot of people will probably skill too many skills to be used, thus spreading too many points into rarely-used skills.
This skill/time management coupled with your build choice will decide on the true efficiency of ones character, not what some people tend to copy from others as they see it works well.
Re:build offers build options to create synergies, cover weaknesses or build upon strengths within the Class tree.
You need to decide for yourself what you want to play and how effective/efficient you want to be.
As resetting will be a permanent feature within the game, make the best out of it and play as you want.