What are the disadadvantages of a datawarehouse with no datawarehouse tables, but only datamarts?
The purpose of the data warehouse is to serve as a single source of truth1 for your BI consumers. It provides you with the central place where the data and metadata for your reports and dashboards originate. If you deprive it of  this role and create a multitude of data marts populated by your operational data (via staging tables or not), inevitably each of these data marts will contain its own &quot;source of truth&quot;, not necessarily correlating with what other data marts think is the truth, thus diminishing the value of business intelligence derived from these multiple truths.
Building a data warehouse is indeed a notoriously long and failure-prone process, but developing ETL procedures is not nearly the most time-consuming part of it; it is master data management and modelling that takes the most time and effort.
Also, the size of your organization is barely relevant; it's the value that you expect to derive from your BI and, indirectly, from the data warehouse that feeds it that should drive your decision to take shortcuts.

1 - Some say it's a single version of truth, because ultimately the source of those data is elsewhere.