Hi Walt-
Great start to a thread. Over the years I've run a world with nexus gates and the occasional star traveler. I can give info on some of the things I've done to rationalize why things are as they are. Part of the answer is look at the framework you want to run and adjust your background. here's a very edited background.
I run a world set in it's own plane of reality, about a nexus hop away from Arduin Prime (Dave's version). As such I can borrow many of the ideas and character types. If you know where to look there are Multiversal Trading Company outlets and a recent assassination was done by the BLS. Characters can venture across the Multiverse to an Arduin shade I run once in awhile. Certain officials have Phraint bodyguards, etc. You might elect to run a shade of Arduin with subtle or not so subtle differences. [For great multiverse ideas see Keith Laumer's Worlds of the Imperium series.]
Why no tanks through the gates? In my realm I decided Gates have mass limits. Put too much through in an hour and the gate destabilizes for a period of time. Force a really big mass and the Gate may go away completely... logistics of a cross gate invasion sucks. And to make it difficult I count the mass in ALL the characters bags of holding, portable holes, etc. Gee, don't check mass and you'll end up falling into the Multiversal void if the Gate goes chaotic... if you're lucky. Some gates will not pass dimensional storage at all... where did the stuff in the pocket go?
I decided on a post industrial/high tech situation and started designing. I have pages of timeline and back story on the reasons and events for the fall and insertion of magic to the world. One issue is that the easy to access mineral deposits are mined out. Even the salvage from the exposed high tech ruins is of limited use as the complex alloys are very difficult to recycle and recover minerals from. My hull metals and ultra-alloy materials have such high energy bond thresholds that even high level alchemy has problems.
So even though several groups in my world have access to high tech info, they are limited by the one to two thousand tons of materials available each year from mining and salvage. Just making the tools to make better tools is tough.
Then there's electricity. I borrowed the energy creatures (Rakshasa's from Roger Zelazny's Lord of Light) who are essentially free willed energy elementals with human level+ intelligence. They love to take over / play with / live in electronics and circuits with destructive results. High level mil spec shielding (Traveller TL-12+ military gear) can resist them, everything else is likely to become scrap. I have a steam punk technology culture with a little weird science as a result. Drag in a radio or electronics gear from off world and watch the fun start!
In the deep catacombs and ruins there are also places where the old tech remains. Imagine the surprise I gave a player who'd come on planet in a Traveller Battlesuit when they opened a stasis vault and the nano-disassembler came out. Quick work by the party saved the character, but his armor was toast.
So as a result there are several know gate locations, all controlled by either a government or commercial consortium. The main free city (T'ell) has several underground levels and in a cavern 2100 foot down is the only known active full gate ring. The city is controlled by a council with a number of interesting factions.
In my case I'm in the process of updating and restarting my campaign, so I have just designed an attempt to disrupt the nexus gate with a Wizard's Wheel (Larry Niven, Magic... series). The attempts to assassinate several major figures had mixed success. The players will re-enter play about 10 years after they last played and in the week following the botched coup. Tons of confusion, paranoia and FUD with lots of opportunity for side jobs and quests.
So lots of cross-Nexus types could still try to invade your Arduin. They're just going to have a rough time. Scorched earth tactics on the Arduin side make logistics pure hell. You can also give a home field advantage to local mages and clerics. Make all newcomers roll against an effects table for the first 30 days or so, before they attune completely. Partial spells, wrong spells, double strength spells or fumbles... make them work for it. Do scrolls stay powered when they go through gates? Only you can say for your world... have fun.
Hope this sparks some ideas. I've had stuff come into play from off world that was powerful, but as the Battlesuit shows there's always a simple solution...