I read in the minutes from a board meeting of The Document Foundation (TDF) the belief ‘plone is a CMS for large scale & for a big company etc.’ and ‘there perhaps suitable’. This seemed to be a lonesome view from the leading body of this entity and not generally shared.
If you take a look at the website https://plone.com you will find there a list of recent Plone launches for organizations and companies. This organizations / companies and the Plone launches are very divers and from small to big sizes, e.g.:
And I found another interesting site for volunteers, that is created with Plone and its new technics: https://vhs-ehrenamtsportal.de/
Thus the view of the TDF board proofs wrong.
The organizations and companies which launched and use Plone site benefit from the excellent stability of this Content Management System (CMS) (information about Plone security could be found at: https://plone.org/security).