I've seen pictures here on ukaps from a fellow with a rather large tank running a sump, he made use of a
circulation pump which you regularly would find in heating systems.
Pumps like this must be placed outside the sump, then you indeed need to figure out some tubing construction like a syphon to make that work since these pumps are also not
self priming you would need to prime it manualy. putting the pump inside the sump prevents this manual priming..
Whit sumps which are not realy for sale anywhere you need to figure out some DIY tubing to make it all work smoothly together.. If you want it all to run perfectly from the start you need to be familiar with math and physics and there are geek formulas to find to compute the tubing sizes needed to equalize pump and drain capacity. Then you need take things in account like, flow restriction of used material, how many bends, diameter, fall speed and pump capacity etc.. If you just go with regular stuf made for an estimated range of capacities like a ready made overflow box suitable for tanks up to 100 gallons then you need to make use of valves to tweak the system to make it run smoothly if you use a smaller volume than those 100 gallons.
That's actualy another disadvantage of a sump, it's not the use of it, it is the need of having the knowledge to build one that rather runs somewhat smoothly from the start. If you have no clue about basic math, physics and available technical stuff you probably wont know where and with what to begin with.
The risk of a pump running dry increases if the whole system is not in sync.. This can be a rather somewhat tidy process since it works partialy with laws of nature which are not realy constant.. Atmospheric pressure can influence drian speeds of the overflow, or tubes getting dirty over time and restrict waterflow and changing the drain speed. If an overflow gets clogged by dirt, chances for this happening increasy again with the need for use of valves to reduce the drain tube size. So building a sump with estimated specifications always needs some tweaking and fidlling to make it run properly in a constant manner. Since it works with forces of nature which are not always constant it needs constant checking and maintenance to keep working properly.. If you by any chance estimated very correctly with the build of your first sump, you are very lucky..
It's a good practice to build in fail-safes like 2 overflowes, 2 getting cloged at the same time are rather low, but not impossible and a floater switch, 2 failing verflows and a failing floater switch are even more unllikely but not impossible.
I burnd out a pump recently.. I my case it happened because i miscaculated my overflows and made them to small (took a risk for aesthetic reasons) and if one syphon breakes the other doesn't make up for it. And one night it happened for no obvious reason and the floater switch got stuck for what ever reason, probably a dirt particle.. So i need to rethink my overflow capacity or construct a more reliable floater switch..
sh*t happens.
With a canister which is a closed system it's not likely to trap air.. All tho if you run it high tech with co2 and oxygene bubbles getting sucked a cannister ca trap air and build it up in the sponges.. This also happened to me once and the pump started squeaking.. So now i regularly shake my cannister and hear the trapped gasses come out. Even sometimes i hear it slurp a air bubble out by itself.. But it's not impossible to build up so much it fails to push it through.. As i experienced it once happening.