safety is first and foremost the most deciding factor
then there are things like:
- time it takes to do the job
- available resources it would take to do the job (ie: the ability to remove any component that would require a crane or other type of lifting device a field tech wouldnt normally have on his truck)
- whether the customer wants it done in the shop or done onsite (see previous reason)

if a tech has the resources to do the job on site that is his main goal but if it is going to take a long time or require resources he just simply cant provide on site then the obvious choice is to send it to the shop. The tech usually makes the initial decision but ultimately it is finalized by his management and the customers decision.

The limitations of what i feel comfortable in the field is determined by enviroment, work area and time to be spent on a particular job. Being tied up for long periods of time just adds to the workload and you will be putting off other customers that might have small problems like a battery, fuse or tune-up to get them back going.There have been places where i have rebuilt engines, replaced sections of mast rails to replacing wire harnesses. I try not to put off waiting customers and let the shop have the major repairs. Most places i go do not have a dedicated site that i feel comfy as to tackle a tranny build and such. I always let the customer make the call whether to repair on site or have picked up. If its a major job i will also mention it will require another tech and possibly lifting equipment. I know there are times when the unit can not be transported from site and no other option but on site repair. Nothing is worse than starting a complex teardown of a unit at a customer to find out you will need a steam cleaner, helping hand ect. It always seems to take twice as long to do a major on site job rather than in the shop due to not having the proper lifting tools or help available. Working out of a van limits what can be efficiently done on site and wasting the customers money on labor or productivity on down time is a bad deal for everyone involved. As the old saying goes " It takes months to gain a customer and seconds to lose them."

Can the work be done safely at the jobsite? Is the customers place of business a shithole with a lousy work area? If the truck is filthy a good steam cleaning makes life easier down the road. If the drive is long how cost effective is three or more trips. These are the main reasons I would send a truck in to the shop.

I used to give the customer the choice, but because of Liability concerns that has changed. As stated earlier in this thread, when a heavy component (engine, upright, counterweight) has be be removed, I take the unit to the shop. There are far too many people here in the US that don't follow rules & I've had the misfortune of having someone almost get hurt by being somewhere they shouldn't.

I find that when I'm on a job in the field, there always seems to be someone who wants to look over my shoulder- which I usually don't mind, but when I have an engine or such up in the air the last thing I need is to have someone hanging around distracting me from the job at hand.

I ALWAYS give the customer the choice, there is no job that -requires- my shop to do it in, but the words I use, every time, are; "is it cost effective".
I have had trucks that they built a cooler around and that truck will only come out when they tear down the cooler wall. I hardly ever have suggested that 5 high full container handler be 'brought to the shop' for a brake job, but I always offer that to the customer who has only 1each 2k to 5k cushion.
There are good reasons to keep the road guys free and not tied up with 6 to 8 hour jobs. Cost per man hour is one. Best utilization of capital assets (the van) is another.
I always use the line that the owner [of the company I work for] says "loaner" is a cuss word, but we have good -rentals- available, and will set a 'max time' limit on how long our part of the job will take, but we sure don't want them to have a truck for free while they decide on added repairs.
All that said, I am who usually makes the call as to what to suggest/sell to the customer, and I have to admit I allow my own workload to fit into where I steer the conversation (if I am really swamped, I will try to send more to the shop).

I will usually give the customer the choice. He usually makes the decision on weather to ship it to the shop or not. I will make most repairs at the customer as long as they have the space and equipment to do the job safely and correctly. I don't let the time be a factor only because if you take a truck to the shop the customer will usually look for a "loaner" truck and if repair is done there then they get to see what it takes to do the job.

A lot of job's can be done on site.

We only normally get a truck uplifted back to the workshop if it's a structural repair ( overhead guard / replacement cab etc. ) OR the customer want's a full professional repaint done in the spray shop instead of us just doing a quick touch up job.

Some customers wont allow mast removal on site so these also have to go back to the workshop.

I have 2 different customers within 500 yards of each other, one isn't fussed if I have to take a mast out on site to repair the truck where the other customer flatly refuses to allow it so truck has to get uplifted.

We take the truck to the workshop only in two cases:
1. Total renewal (for example as a preparation for the further sales)
2. Engine main repair, when we need to check the truck after it.

Other cases, including the transmission job (hydrostatic for Linde), we do on customer site. The technicians are sufficiently equipped and technicaly prepared to do it.
We do it to reduce the repair time, raise the first-fix factor and reduce the costs (truck transport to the warkshop).

Sometimes we take to the workshop only some assemblies like electric traction motors, or attachments.

Usually, even the complicated work on site stops the truck for maximum 48 hours, usually less.

We find, that customer appreciates this.

Sometimes the customer ask us to remove the truck from his site for the repair, but it's rather rare situation (costs, prolonged time etc)