As I cannot state precisely which insurers are refusing to cover self-declaration medicals, you are entirely at liberty to consider my statement as hearsay.
I have never advised any club to refuse to rent an aircraft to a pilot who does not hold at least an NPPL medical declaration. It's up to them, but I can see their concern whilst the situation remains unresolved.
However, I have certainly advised individuals to steer clear of the self-declaration system (except for legacy UK PPL / NPPL holders flying non-EASA aircraft) at least until the CAA has clarified the actual legal situation, rather than the syllogistic assumptions in which some believe.