Well for starters the cfs4
Has very thin cheap plastic parts, I was always afraid to clean it because I was afraid of breaking something. I wish I could tell you what happened with it but it just stopped working one day and had leaked all over my floor. I suspect the leak caused the motor to run dry. If they work good for you that's great but I won't buy them anymore. It's not worth the trouble of sopping up many gallons of water from my floor.
WOW! I can see how that experience could turn a person off about the product in question! I often fear noobs will have a bad experience and it will influence their opinion of externals forever.
However, plug in the search terms, fluval and leaking, or ehiem and leaking, and google will return thousands of results. With canisters, leaking is always a BIG risk, in my humble opinion--in any case, it is a worse risk than HOB's or internals. This is an area where HOB filters come out a head. About all that can be done here is care in reassembly after cleaning and checking THOROUGHLY for leaks before trusting it. Indeed, another check 1/2 hour or so later is an excellent idea.
I have seen it all with my wife. I don't let her touch my tanks/equipment. Water on the floor from improperly maintaining a filter used to be quite common with her! Indeed, I have caught her leaving parts off is she didn't know where to put them back!
Another problem I have heard of is that, newbies tend to over-tighten fittings and bust them, loose an important o-ring in the cleaning, or are just too "robust" in the handling of them and end up breaking something. Or, they don't take care in keeping important contact areas, which provide the seal, and keep these areas clear of debris; a sharp small object, such as a sliver of aquarium gravel, etc. can get on these areas, if these joints are then forced together the watertight seal can be compromised. (yeah, wife has done that too.)
And of course the materials are a bit cheaper! You can have 2, 3, 4, 5 or more of them for the cost of one of the expensive ones! But, you are quite correct, in my experience with running them, the cost savings alone make them my first choice. The LFS here gets direct shipments of pallets of this equipment. Since we have the Port of Stockton here (farthest inland shipping port in the USA, perhaps the world!), shipments from China are cheap, fairly quick and easy.
Indeed, I would even recommend them to newbies; I mean if they are going to make a mistake, or ruin a filter, I figure they will end up thanking me when they only do it to a cheap one! However, noobs might want to set the filter in a dishpan until they gain more confidence in their experience and/or the filters operation and proper care and handling.
Regards,
TA