50MHz SPI PCB routing, use vias or resistors?
SPI bus at 50MHz can easily run a couple of inches thru a few vias without hitch. Wavelength of 50MHz is 6 metres but realistically because fast edges are used you need to think ten times faster. Even so that's a wavelength of 60 cm. Rule of thumb is keep tracks smaller than a half of a quarter wavelength (other folk will use other rules of course) and this means 7.5 cm or 3 inches.

Try and keep clock and data same length and if you hit problems on long stretches you might have to terminate at high frequencies with 10pF in series with 100 ohm (more likely on clock than data).

There is no great theory in my answer, just a whiff of theory and some experiences and he odd rule of thumb. Bottom line - use a ground plane - it's no excuse to say you are not familiar with grounding - get familar - it'll be the difference between something always working and something nearly always never working - it's that big a deal.