So far it has not been an issue. In five years of building sites this way, I have not had a single case where the font rendering was noticeably off.
There is one big fat glaring exception: Safari for Mac doesn't resize fonts exactly, but instead uses a series of steps.
This means that when the page is resized, the text changes size slightly. I had to do a little back-end magic to compensate for this.
You can see for yourself by opening this in Safari and resizing the window:
‹html›‹body style="font-size:5vw"›‹div width=100%›
Slowly drag the side of the window‹br›
and watch when the font changes size‹br›
‹br›
In Safari it jumps, in any other browser it's smooth
‹/div›‹/body›‹/html›
You'll have to swap out the tags — I replaced the real brackets with French single quotes to make it appear correctly (possibly not necessary).