We've been experiencing similar issues - with ad-hoc reports as well as subscriptions. Even for extremely simply reports that analyze email sends for a single month. I opened a ticket with Support and was told the following:
The reason that subscriptions fail to send are because they timed out - the usual cause behind this is that there's too much data. A secondary reason is that there's not enough processing power to be shared with processing the report ... Revenue Cycle Explorer is a 3rd Party OEM that Marketo is using. There are limitations with using this reporting, and Marketo is unable to add additional resources to this process because it is of a 3rd Party.
Support often suggests "workarounds" like limiting our filters and/or reporting period - to as little as a week - and piece together the data using Excel. I was pretty flabbergasted by this response.
I always thought that Marketo acquired RCE from a 3rd party and integrated it within their platform. I didn't realize that they were using a white-label service and branded it as a Marketo add-on. Nevertheless, I find these responses/reasons unacceptable - especially since we are paying extra for this add-on capability. I really hope Marketo invests the necessary resources/costs to develop a more capable advanced analytics capability for those of us that need - and pay for - reporting/analytics beyond the core platform.