Used to crash at 3 settlements. But game has been pretty stable for me since the 3.0.2 release. I don't know if it helped but after upgrading all my addons game has been pretty stable.(I use Mod Organizer so it was a painstaking process of manually download, uninstalling and reinstalling). I also cleaned out my mod list and removed homemaker plus other build mods.
That said, I am more careful now I only run two settlements at a time on autobuild, wait til level 3 and then move the companion. So far, I have developed Oberland Station, Jamaica Plains and Croup Manor this way. If your system is too low, I'd avoid developing big settlements like Sanctuary or the Castle. I have been consistently crashing even by just walking around a level 0 castle on separate saves. I attribute this to my hardware. So you'll have to try experimenting which settlements are stable. Save before autobuilding a settlement or if you're adding a new settlement to autobuild so you can easily roll back.
I don't know which mod I removed that helped stabilized my game but here are some of them that I removed
All Settlements Extended
Altairp's Animal Farm
AS Craftable Institute Stations
BanCo- Bank of the Commonwealth
Better Junk Fences
BlueBeers Water Purifiers
Busy Settlers
Craftable Armor Size
Crafting Fury 9000GTX
CROP SE- Create your own Planter
DC Guards Flirt with Both Genders
Dinomore v1.1
Director's Quarters
FCOM 2.7
Homemaker-Expanded Settlements
Homemaker- Unlocked Institute Objects Patch
Housekeeping (never got it to work)
ICS wifi router generators and such
Immersive Settlers
Legendary Modification
Minutemen HQ- Museum of Freedom
Modular Kitchen
Multiple Floors Sandboxing
Northland Diggers
ALL New Settlements (Like the Natick Powerplant, Fiddler's Green, etc)
Settler Sandbox Overhaul
Sim Settlements-Wedges of the Commonwealth
Singing Settlers
Snappy Housekit
Snappy DL03
Thematic and Practical
Workshop Planters Fixed
Workshop Rearranged (removing this significantly increased the load of opening workshop)
There's more but these are what I think are most relevant to settlement building.
I recommend giving your mod list a slight diet. Over all from a 158 mod count, I now have around 60 mods.
System: 8GB Ram, i7 1.8Ghz, Nvidia Geforce 760M, HD not SSD.