Ultrasounds are relatively inexpensive compared to other imaging tests. I'm pretty sure that is why my doctor ordered one instead of what I asked for.

As far as the reliability, they won't be able to conclusively diagnose anything with the ultrasound, but they may be able to see if something is there that is concerning. I had my ultrasound last week. I have a lump that is mobile and rolls between the front and back of my lower jaw bone. It feels much larger than 1 cm. I have a similar one on the other side that is a bit smaller. My results showed that lymph nodes were present all under 1cm. This leads me to believe they weren't able to see the lump even though they extensively scanned that area on both sides.

I think the US is an okay starting point, but if they can't see anything and you are still concerned, keep pushing for answers.