You are on the right track with your assumptions.
Advantages:
1) Immune to EMP/electric surge:
In general day to day use, yes, especially when compared to say a hard drive. As for the EMP part (hopefully we never find out), it would depend on several conditions, though it would most likely survive much better than a HDD
Disadvantages:
1) Slow:
Depends on your equipment, particularly the speed of the Blu-ray optical drive being used. Wikipedia has a nice summary of speeds:
https://en.wikipedia.org/wiki/Blu-ray_Disc_recordable#Speed
Hard drives and SSDs would most likely be faster.
2) Limited space:
Yes, Blu-rays have limited space per disk but so do any storage devices. Compared to modern consumer HDDs/SSDs the space per Blu-ray is relatively low, however, this is not really an issue as Blu-rays are generally significantly cheaper than a HDD/SSD of the same size.
3) Only write once:
If you use a single write Blu-ray, yes. Re-writable Blu-ray discs are also available.
This site is a good summary of the pros and cons of backing up to Blu-ray:
http://www.databackuponlinestorage.com/Blu-ray_Optical_Discs
As stated per the link above, it is difficult to accurately state the life expectancy of a Blu-ray, although theoretically they could last 50-200 years. Practically, their lifespan is dependent on how long the Blu-ray standard is supported.
Like any optical disc, there are three primary dangers:
- Direct sunlight
- Excessive heat
- Scratches
Scratches are easily avoided by storing them suitably, usually in a high quality 'disc wallet'. Heat is avoided by storing them in a relatively cool location, ideally no warmer than room temperature.
In terms of extreme surviving an atomic blast or something extreme, it's not very likely. Perhaps they would survive a flood for example, assuming they remain protected in a case. As for other disasters, chances are low.