No One Person has been able to hear in use most of the Single Driver Speaker Designs that are in use either from a Commercial or DIY Build background??
For an individual to suggest most Single Driver Speaker don't sound very good is a fantastical statement made without a solid foundation, as a result of their exposure having very very limited experiences.
At best most of the ones heard by an individual, especially an individual who expresses a negative content as their evaluation. Might have been exposed to an End Sound that to them did not sounded very good, if that is an experience had, then I am not to argue.
Any individual is limited in what is able to be experienced and only able to encounter what is a very very small proportion of the entirety of single driver speakers.
Which as a overall quantity, adds up to a minuscule amount of speakers that are actually available to have their End Sound heard and assessed.
Where the evaluation by an individual of such a small amount of speakers is that all Speakers using Full Range Drivers are no good, is a statement that is extremely deficient in information.
I would strongly suggest such a statement is bordering on worthless.