This is the thing that makes me hate multiclassing as it currently exists in 5e.
It is a tool for people who have system matter to "generally be better in terms of power," while people who are just noodling with the system or trying to realize a character concept usually fall into the "detriment" trap because they're not actively looking for synergies and trying to maximize their power.
I don't love D&D as a game of finding CharOp cheese, of seeking synergies, of tactical character building. Some folx absolutely play for that, and I'm glad the option is there for those folks, but I find that the existence of multiclassing really distorts the design environment. New material must be designed very carefully to avoid giving people who multiclass a remarkable edge. Whenever someone in my games wants to realize a unique character concept, I would MUCH RATHER they bring me a 3rd party subclass or work with me to make a new one than try and cobble together Frankenstein's Multiclass. Either they know the system really well and will end up generally more effective, or they don't know the system very well and will end up generally less effective, and either way, it impacts the group dynamics ("Why is your warlock/fighter more effective in melee in this moment than my character who is Just A Barbarian?")