Wow, you must be doing something wrong. Perhaps crappy PSUs? I just got my 4th UPS in about 10-11 years (I get a new one when I move to a different Voltage country, or the battery dies), my company is in their 2nd or 3rd batch for the same duration, and I've never seen what you describe. In fact, I have lived in places where we would get brownouts several times per day and without a UPS there was a reboot each time.
Funny story. The first time I got a UPS was when I went to NY for grad school. I ordered all parts from NewEgg, assembled a machine, hooked it on a UPS and the same day I had arranged for the cable internet to be connected. So, I was all up and running and I distinctively remember saying "perhaps the UPS was overkill for NY, I don't suppose they have such problems here" just a few minutes before the power went out! It was August 14th 2003, and the power went out for several hours over NE USA & Canada... No, my PC did not reboot.
But, yeah, if you say you get good UPSs, it could be the PSU. I always get rather expensive units from the likes of Antec.