Selecting the right tool for the job can be a very time-consuming task in Drupal development so I felt I could post some personal experience that may benefit some people coming from Google.

My goal was to simply import a hierarchial tree of
Countries => States => Cities
into Drupal that could be selected in a hierarchial fashion to identify a city. I elected to give Simple Hierarchial Select module an opportunity to automate this feature. While I initially downloaded "location taxonomize" module to synchronise newly entered addresses with the vocabulary I don't think it will be needed for my initial site development and I won't be using the address module anyway for now [without validation you end up with duplicate wrongly spelt city names anyway?!]).

Since I was playing in Drupal 7 where taxonomy terms are treated as entities I also decided to add some information to the cities, notably the latitude and longitude info already available at hand, which I could stick into a geofield (field).

While Taxonomy CSV import/export was fine for the simple task of importing a hierarachial tree, I understand that adding data to a geofield, for example, is a more complex task. Note that I haven't really tried hard but since I found an alternative I stopped trying.

I used the Feeds module instead to achieve my intended goal in a single import. It's not the most intuitive module in terms of labelling exactly what the buttons do but appears to be supported by a lot of third-party modules (including geofield).

If any of you have ever used a CMS like Prestashop you may be familiar with importing hierarchial categories using parent-child relationships. In my case, a country, state or city would all be assigned a unique identifier (or 'GUID' in Feeds). The countries don't require a universal parent identifier in Feeds (as far as I know). All the states will also be assigned a parent country's GUID and all the cities a parent state's GUID.

If you have a huge global list from somewhere (e.g. the UN's list) where you'll have multiple cities in a country (but hopefully never more than one city in a state!) you overcome this by concatenating and grouping your list first e.g.
Springfield, Maine, US
Springfield, Maryland, US
Springfield, Massachusetts, US
Springfield, Michigan, US
Springfield, Minnesota, US
Springfield, Missouri, US
Springfield, Nebraska, US
Springfield, New Hampshire, US
etc etc etc etc etc etc............................
should all be unique cities.

Maine, US
Maryland, US
Massachusetts, US etc.......
should all be unique states => you then removed duplicates from this column in Excel (or LibreOffice etc) before assigning GUIDs to all states in a new table and vlookuping the GUIDs back into the original table. Countries of course should be unique already.

If you create a Geofield in your vocabulary type you can then add columns/fields in your CSV to represent latitude and longitude and import all required information in one hit.

I haven't fully tested all the options yet but just thought I'd put it out there.

To be frank, Drupal isn't natively setup to expect 80,000 hierarchial terms in a vocabulary and you'll most likely get memory errors trying to access page 500 of your vocabulary and slowdowns whenever you try to access your tree (especially after cache is cleared, as expected). I haven't benchmarked whether the taxonomy edge module gives any speed benefits yet. Customising a database with similar design and/or mathematical principles in mind I guess may be the fastest option.