Depending on size thermal expand and contraction does come a factor in play as well as you scale up but there are other things about rack and pinion.
Note the Lubrication is key section on rack and pinion to avoid excessive wear. PatrikL the main reason rack and pinion systems get dirty is the lubrication material glues what ever floating dirt to it yes upside down mounting helps a bit but does not solve it.
Belt systems avoid the Lubrication issue but they normally suffer from more thermal expand and contract issues as they get longer.
RichardJHauser single belt based systems as size expands can have more slop than rack and pinion due to thermal expansion and thermal effects on how much belts stretch. Duel belt systems can run into issues with teeth not meshing if fixed belt and the drive belt end up at different temperatures it is why the patented method has dual belts running complete length. Thermal expand and contraction becomes a bigger and bigger nightmare as you scale up. Tank tread style might cause your more problem than you can dream running on a fix belt that at times going to be different temperature than the track tread belt . Tank tread on a smooth track would have less thermal expand nightmare to worry about question is can you get enough traction to get exact movement…
There is another question with how accurate a farmbot needs to be does it in fact need belts in the small size or could farmbot use a spool drive and heavy duty fishing line. Stuff like fishing line is way cheaper to make and acquire than belts or racks. To be stupid some farm equipment used hoses on spools as lines to pull quite large machines around.
Basically every bit of toothing be it belt or rack equals a increased cost compare to using toothless. Question is will toothless be good enough. All the prototypes of farmbot I cannot see any tests using toothless.
With racks there is a lot of steel racking out there that will survive UV but they are not rust proof. Even the stainless steel ones will slowly rust out in the environment. My problem is I don’t know of any rack that if you look closely is rated for where the farmbot operates. I can think of many toothless options like heavy fishing line that are UV rated and would be fairly cheep to replace if there is a issue.