Location based mobile app and Drupal backend- opinions?

I recently developed something very similar for site engineers at a large pool maintenance company.

The first issue is connectivity... no signal, no site :-) A native app with an API to your Drupal web application would solve that or it may not be a priority issue in phase 1 of your app build. (It wasn't for us)

Otherwise, we just used content types for the forms, since the Drupal Field API is in core. So any image, text, select fields we needed are easy to add through the UI.

Field Collections module gives the ability to add multiple records of data to a content type, such as parts needed (with costs, qty, tax sub-fields), etc.

Views module - you can create reports such as new job sheets, engineer reports, client feedback forms (all content types)

Field Workers and Customers can have their own user accounts and User Roles. User Roles allow you to control which content types a Field Worker and a Customer can view. A small amount of custom coding and views configuration makes it possible for Customers to only view their job sheets and reports. So no install for different teams would be necessary (rarely is that justified IMHO).

This is actually rather simple stuff IF you are very familiar with Drupal. I probably only wrote 100 lines of code in a custom module. You could get a prototype setup fairly quickly with limited Drupal knowledge. To build this on any platform you're not familiar with will have a learning curve but certainly Drupal is perfect for this sort of app.

In terms of 'is Drupal scalable in this type of application?' - Drupal can handle hundreds of thousands of data records/nodes/user accounts or whatever. There are advanced search and caching modules that integrate with technology such as Apache Solr, Memcache, etc.

It should be possible to whack up an install and get a fair amount of functionality in a prototype without writing code :-)

Best of luck.