Difference between viral and native RNA
There is no fundamental difference between viral RNA and and native cellular RNA other than the sequence of RNA bases in them.  The sequence differences are not biochemically apparent in the RNA, only in the protein products produced from the RNAs.

Regulation of native RNA is done in a huge variety of ways (nothing in the cell is designed; everything is ad hoc, so any random happenstance can become established).  Native regulation is essential, otherwise runaway production will happen leading to cell death or to cancer.

Much, if not most, of the native regulation is done by regulating transcription of DNA into RNA. Once the RNA is in the cytosol, little further regulation is done, so viral RNA in the cytosol has the run of the cellular protein factory.  But at least some viruses have acquired further advantages which help their RNA out-compete the native mRNAs for use of the protein machinery (see, e.g., Hijacking the translation apparatus by RNA viruses)

So it is not really an easy thing to "label a sequence of RNA as dangerous simply because it is unregulated" as the wide range of normal RNAs in the cytosol does not provide a basis for determining if any are "unregulated" and the cell cannot be protected in that manner.

Self-assembly is widespread in normal cellular functions as well as in viral production.  The molecules involved have long-honed attractive points for their mate molecules, and in the jiggling molecular soup of the cell the molecules can soon bump into each other and join up.  Thanks to the virus's uncontrolled production, new viral particles have plenty of component molecules available for this assembly.