I used my 901 as a basic home server for a while - it certainly worked fairly well... until it didn't. My issue was the SSD lifespan - unless your 901 has a Hard Drive (which I believe was only on the 900), these old 901's had two SSDs, neither of which has a particularly long lifespan, but particularly the larger MLC unit. Certainly not the number of read/write cycles you get from a newer SSD.
Other than that, your basic problems are performance and heat if you run much above idle - they weren't designed to be always on, nor run flat out, and can get fairly warm after a few hours of use. I believe this may have contributed to the death of my SSD.
As for performance, they don't have much RAM, what they do have is slooow, and the early Atom processors don't have much power. You're limited to 100Mbps ethernet, too.
Storage is definitely an issue - the internal SSDs are slow, at around 20-30Mbps read speed. The SD card and USB slots are all USB 2, so are really no better.
I very quickly moved all my media and file storage use away from the unit, and never even attempted to do anything really intersting with it, as it was quickly obvious that it wouldn't work although if you're just wanting it to be a basic VPN, DNS etc unit then it will do the job.
And finally, they really aren't that low power at 36W considering how puny they are. I ditched mine for a HP Microserver, which is significantly more powerful, much more flexible and only a little more thirsty at 50W.