Can you describe the issues with single class warlocks that are serious?
The "Well you could just use the SA/advice from crawford's 5+ year old tweet" is one people in this very thread have repeatedly brought up is an example of a non MC problem. The fact that people were talking about it long before I even posted to the thread lends weight to the need for its inclusion in errata.
On the MC front... Of course WotC gets credit for the completely unforeseeable need to actually
errata "Eldritch Invocations (p. 110). The following sentence has been added to the end of the first paragraph: “A level prerequisite refers to your level in this class." that absolutely nobody could have predicted as problematic.... but they lose any good will they gain in the process & more for not doing the same kind of clarification of what "spell slots" are covered with Page107's
"You regain all expended spell slots when you finish a short or long rest".... what 12 dimensional chess champion could possibly predict that one. That chest deep hole digs itself across the planet when you consider the utterly predictable attempts at coffeelock nonsense that resulted from Aspect of the Moon in XgE. That hole goes on to through time & space when you consider that every cantrip in the game scales by adding extra dice rather than attacks, every extra attack class feature includes words like "in this class" after some of 3.5's lessons,
and Eldritch blast was changed from a class feature in 3.5* that scaled based on warlock level to the only cantrip in the game that gets extra attacks along with the ability to add a statmod to those attacks among other things.
On its own warlock isn't problematic & generally interesting to have at the table on the
very rare(bordering on legendary or artifact levels of rarity tbh) case I see one that isn't just low tier1 & not a scorlock
yet, but there is too much uncorrected "oops" on par with bad homebrew on WotC's part to be coincidence & I'm quite sick of the player vrs gm bad blood/animosity that results in needing to make these rulings again and again at the AL tables I run at a local FLGS rather than just saying "here's the errata, don't keep being mad at me for WotC's mistake".
Yes multiclassing is "optional" and a GM could deny all multiclassing, but that's an absurd suggestion that admits warlock is so
deliberately broken in a MC environment that it alone causes MC problems.
* 4e had it as an at will power & didn't really have multiclassing so much as a handful or classes that pretended they were multiclass so it doesn't matter what it was then.