The failure to protect accessible toilets in the same way as the sign protects single-sex toilets is a big disappointment - otherwise this sign is great, and should be used everywhere.
Some mental illnesses can indeed be disabling; however, just having a disability does not automatically mean you should use the accessible toilet.
Many people with disabilities are perfectly capable of using the 'ordinary' toilet. For example, the fact that I am deaf does not mean I need an adapted accessible toilet, so I leave it free for those people with disabilities who don't have the luxury of choosing which toilet they feel more comfortable in - for some disabled people it's the accessible loo or no loo at all, it's not a choice.
At times my other disabilities limit my strength and mobility to the extent that I physically need an adapted accessible toilet to be safe, in which case I use the disabled toilet. But I only do so when I actually need to.
Disabled people know that we are not all one homogenous group who all need adaptations to make toilets accessible for us, and we know the importance of keeping accessible toilets free and available for disabled people who need the adaptations.
There is no way of policing women's toilets to 100% keep men out, it works on trust - 'the good men stay out so the bad men stand out' .
Similarly, the decision to use the accessible toilet is a matter of trust and personal judgment, but it is every bit as important to state on signage that the accessible toilets are intended for people with disabilities only, as it is to state that the women's toilets are for biological women only.