I have never heard of Velotooler. The mobile service I have heard of is
Velofix. That is not an endorsement; they were simply the first to become national (through franchising).
Most bikes are 80-90% assembled. You install the front wheel, the pedals, the seat and post into the frame, and the handlebars. Then it is "ready." Some companies factory assemblies are better than others. Regardless, whoever you choose may elect to install those few item and take your money, while others will take an hour or more and actually confirm that things were assembled and adjusted properly. The only way to know is to watch over their shoulder, so to speak.
I personally have assembled thousands of bikes. Most all bikes need to have the bearings adjusted (sealed bearing excepted); they are almost universally too tight, which causes excessive drag and wear. Wheel spokes need to be stress relieved. Cables are often fitted dry and need to be lubricated. The cables are never pre-stressed either, so that should be done as well. Sometimes the factory puts on too long or short a cable, and that needs to be corrected. Every fastener should be checked; especially crank bolts and the ones holding the saddle or adjustable stem.
Ever see a bike with a reflector pointing towards the sky or ground, or no reflector at all? It is because the person who put it together never bothered to do it right. Ever had a water bottle or rack bolt stuck in a frame? Blame that on the assembler; those bolts should have been removed and coated with grease or anti-seize before it was delivered. That is the level of detail that separates a good build from a mediocre one.