That doesn't really hold when their competitors have teams of engineers working on DDoS mitigations and successfully handle most of these attacks. Falling to a DoS is a technical failure just like other causes of outages.
I looked into blackphone a while ago. If I remember correctly the first thing it asks for is your e-mail address so it can sign you up to their proprietary anonymity/cloud service. I'm not sure if this is still the case, but if it is I think they've completely misunderstood their target audience. Thanks for reminding me though I will have another look into it.
I'll stick with Fastmail is very reliable and independent. They can't really be blamed for a DOS attack.