Liesje
First off, I'm not bringing this up because I have a problem with contracts in general or contracts between my breeder and myself, this is just something that was on my mind today. In college I took contract law b/c I was interested in it but never pursued it. So I know some of these cases are not black and white and take a lot of analysis.
First and "bottom line" answer is it completely depends on the state and how well written the contract is. And THAT depends on the breeder and how much they are willing to spend on attorney fees (and trust me -- the more lecherous ones are often the more 'well protected' ones).
They come from different standpoints -- and again the personality and ethics of the breeder come into play here because are they so completely convinced they are "on the dog's side" that they are willing to literally barge in, and bulldoze their way thru and snatch the dog "as their right" (picture crying children or lots of screaming or even letting the dog out of your yard) Or are they going to go completely within the laws of the state/locale and go thru all the various ramifications of whatever your area requires as far as repo'ing property (much like reposessing a car -- we're talking "possessions" here -- not usually going to be a concern that this is a living entity) -- again that's totally going to be dependant upon local law and how the contract is written.
OR does the breeder simply want to be able to have basis to sue -- and some breeders are lawsuit happy -- mostly frivolous, incendiary suits designed to slander within the law and cost the owner TONS and BOODLES of money just to fight suits, take off work, etc. and essentially bully people into standing down and either paying "damages" or relinquishing the dog.
Liesje
What can the breeder do? If the contract states consequences of not abiding by the terms, does that change anything? For example, say in the same situation the "contract" went on to say that failure to neuter the dog within 4 months would result in the dog being re-claimed by the breeder. Can they really dog that?
Sure they can DO anything they want essentially -- and this is where some people wind up in these long-drawn-out-never-ending-costly suits that become he said/she said things. They may bulldoze their way in and snatch the dog (let's wait until the wife is home alone or the kids are being watched by a sitter) or they may suffer thru endless legal process to get a Writ of Possession (and if they drag the owner thru enough endless court appearances where someone has to take off tons of time from work -- most folks will just give up rather than lose their job for excessive absence and bad publicity).
But I'm going to go out on a limb and say yeah, a lot of this is threatening jargon, but the vast majority of it may simply result in lawsuits based on the failure to abide by the contract and whatever "damages" (monetary damages) they can trump up. Most of those damages are going to be things like lost work and vet bills, etc.
Essentially people can "do" anything they darned well please, and then someone else can try to sue them for it. How those things *should* go and how they DO go may be 180 degrees apart.
A few weeks ago someone posted a news blurb about this guy in Tampa, FL who "got thrown in jail" because he didn't re-sod his lawn and keep it nice. I've actually talked in passing to several attorneys about this since just to see what they'd say. ON the outside it SOUNDS LUDICROUS that anyone may actually have been thrown in jail for such a silly thing.
BUT the bottom line is, some attorney for the association was dogged enough to keep on going to get the judge to sanction this guy one more time (after a zillion times) and the judge finally got tired of the guy not showing up in court and the JUDGE put him in jail. It never was the Association - it was an offended judge.
Same thing but different -- in that crazy Senator Stevens thing in Alaska -- one of the JURORS claimed she couldn't come in because her father had died and she took off DAYS for "his funeral". Eventually they replaced her with an alternate just to get the case back in the courtroom.
THEN they discovered (somehow someone ratted or she admitted it or something) -- she didn't lose her father at all. She went to a HORSE race!! She dissed court to go to a race because she had non-refundable plane tickets.
How dumb can you get? The court coulda/shoulda put her butt in jail for that. It's called "contempt of court". And lo and behold that judge just plain let her off!!
Difference in judges. Difference in courts (county vs. federal and the federal judge had WAY more leeway to do her serious damage).
So how enforceable? That's something you'd have to evaluate on a case by case basis with the particular, individual contract and an attorney who knows local/state contract law there. Frankly, you can cause someone just as much damage as YOU are willing to cause and create. Does that make sense? IN other words, it likely costs $250 or more just to file something in your county "small claims court" (here in the States) and that's JUST to hand the paper to the clerk. Someone either has to do a ton of research online to get the forms TO file, or pay an attorney to draw up a suit ($250 or so per hour -- not just the suit but the summons, court's cover, Lis Pendens, etc.)
In short -- suing someone costs money. Lots of it typically. so the route most people might take would be to go and bulldoze their way in to GET the dog back and then let YOU sue them to get it back sort of counting on the fact that "they had a contract" to intimidate YOU enough to not sue them (counting on your love of the dog or their contention that they can do better by it and let the judge sort it out). In other words, just because THEY have a contract you signed doesn't mean *they* will be the ones to sue you to accomplish that thing. They'll probably take whatever steps they assume the contract gives them the right TO take and let you be the one to spend the money to sue.
I'd never walk forward into an undertaking *assuming* a contract was unenforceable because typically if someone is serious enough to require a contract then somewhere they are planning on making you miserable enough to make it stick. Most breeders, etc. aren't going to go to the $$ spent of hiring an attorney to sue you to get the dog. Probably not in the 'real world'.
But I have known lots of good breeders AND good shelters/rescues who have been sued and lives ruined over it. And I've known some nasty ones tho protect their nastiness by nastier contracts that make it too costly for anyone to fight them (and they usually have some attorney on retainer to back their play).
Most folks assume, as you rightly said, the because there is a contract no one is going to break it. And most of the contracts I've seen regarding dog stuff are ludicrous at best. The better ones are usually the unscrupulous idiot who wants to protect his back. Isn't THAT a scarey thot?