Used these for around 1 year now but still not at the point where they are set and forget.
First issue is runaway res.
I personally use multiple smaller reservoirs (In fact I use 1 res per pot). The small reses are basically 5L buckets custom fit per 1 blumat. If one runs away, as it has done 2 times, then you are only dealing with 5L of flooding, my water catch system (gutter system basically) can catch that. If you use one huge res with 100L or so you need to be able to catch that amount of run off and it is almost CERTAIN to happen at some point. Get a system in place to deal with it or use smaller reservoirs if you logistically can not. The smaller resses come with some issues, like the pressure rate isn't as high for obvious reasons. To counter this you will simply end up having the blumat dial open one extra notch (example) during the dial in period.
The downside is that they need filled up every day or every other day depending. This is an easy task with a watering wand and bilge pump connection from the main ''slave res'', the watering wand also plays well with any manual feeding you may have to do, so long as you time how long it takes to fill 2/3/4L or how ever much you need. All in all it creates extra work over a 100L+ res but you have a lot less risk of flooding as an up side. Other upsides is if you do have a run away, you only lose 5L. You will be thankfull to not have to clean up 100L and then also have to replace that amount and reset every single blumat from scratch. Thirdly, having to fill up the small reservoirs every day/other means you are always checking on them. If you use a 100L res and start to get lazy since it can last a week or so, you may have a run away when you've decide not to check for a few days. The plants will be dead if that happens. They get use to continuous amount of water and even 1.5 days with none will have a huge negative effect. You can see with blumats that roots will grow in a huge mound out of the coco, even as high as 4 inches. If those roots dry out at all (takes only hours), they never recover, your yield will take a hit. This has happened twice on separate occasions and only two pots suffered, one catastrophically (imagine that was a full linked system).
Here are some tips on avoiding run away res. Make sure your pots are on even flat ground, if this is not achievable then get a small spirit level that can give you the slope angle on the pot. You want to have the blumat on the low side of the sloped pot (even a slight slope). The drip hose is mostly good to be about 3 inches long and have it pointing toward the high side of the pot. Due to the gravity/drip pipe length this will better ensure that water is making it's way toward the carrot so that it gets wet and actually shuts off. If you have the blumet on a high spot and/or the hose is too long facing low spot the water dripping will not spread to the carrot enough and you will very likely see a run away res.
Second thing is to make absolutely sure you wash blumats in clean water and THEN put them under another set of clean water before you begin setting them up under water. You don't want any tiny bits of old soil/coco finding it's way into the valve system.
Next issue is keeping things clean.
Make absolutely sure you clean all lines and res to a clinical standard between every cycle. If you fail to do this sludge will begin to build (particularly in the res and main line). It won't be all that visible but it will make it's way down the small blumat pipes over time. What this leads to is a lower flow rate, you then have to open the blumats more to compensate, the pipes can clear under the added pressure (so I theorise) and then you end up with a res dump. If that doesn't happen, at the very least inconsistent drip rates arise blumat to blumat where some are bottle necked and won't get enough water>yield.
Drip clean is a useful product that definitely works but never use it on the fly. Only ever use it after a good clinical level clean out or you will run a real risk of blocking the tiny blumat lines as drip clean really does pick up all the sludge (first hand). You can use it at 1/4 rate if you are only using plain water, this will be more as a ''just in-case''. If fully synthetic and using what you know is a clean nutrient (like jacks raw salts) then you will be able to go with 1/2 strength drip clean again as a ''just in-case''. I use full strength as I have my own reasons to do so, such as my bilge pump leading from my main res that I use to fill the small reservoirs. The pump can gather some build up in the pipes so I use for this reason. Also keep your res/reses sealed as tight as possible to keep contaminants from falling in. Reses/pipes should also be completely light sealed for algea build up, ignore that at your own demise. I have a separate drill pump for clearing any water that may find it's way into the catch gutter system. The pipes on that drill pump are half see through, they are completely green on the inside. Algea evidently has a high tolerance level.
Neutrient strength has been the biggest challenge. When you get it wrong it is difficult to correct. I won't even pretend I have it figured properly yet although I feel about 80% there. To get a real understanding of it requires you to know the minimum ppm values of plants, and then you need a little bit trial and error to translate that into a blumat system that doesn't have run off.
The flow rate of your system matters, as will your temps and light distance. It isn't an easy thing to dial in properly since your pressure rates from res will effect flow rate (as in my arrow positions won't be the same as yours) along with different neutriant types/ratios etc.
This is something I am still trying to figure out to become consistant. In truth it's hit and miss at the minute. When they work and get dialled in you end up with 4inch high root mounds and crystals/density up and down the plant like you've never seen before. If you are having problems for what ever reason you will wonder why you bother, since the results are at best on par with manual feeding+the hassle/negative feelings of knowing you didn't get it spot on this time.
My advice for anybody trying this is to only do it with 1/4 of your pots for a few runs. You will run into trouble that much I am almost certain of., but at-least you won't be risking it all. I still don't run it fully, I do it on half and will not go all in until I get 2 in a row of no issues. Keep this in mind as it's no understatement, if you have a runaway linked res just after a top up you could lose them all if you are not doing daily checks. If you end up with neut issues they are very difficult to figure out and fix.. as they could be a combination of things due to no run off.
On a side note, anybody who knows what ratios and elemental ppm values they are running please share.
Mine are: Elemental ppm
N - 150 (reduced by half after week 4 or so) started seing slight N tox with no run off otherwise.
P - 38 in V 60 in F
K - 160 > 189
C - 95 > cut in half same time as N
M - 60
S - 65