Here's my take, everyone likes to use the extreme views on the other side to argue against, and while there are a few people that seem to believe those, they are generally not people with more then a few months training.
From my point of view:
MMA is a competitive format, in the old days it had no real rules, apart from the environment. It started a system of showing what did and did not work in that environment.
It may not be real, and has grown even less real, but, there are fairly good methods of testing what is and what is not a reliable technique in a restricted arena.
Yes, it is not "real", neither is a wind tunnel or computer simulation, but if the plane crashes in those I wouldn't want to be onboard for a real test flight.
Training, is like the simulations before a test flight, on a plane we will likely never have to fly. We can get get close to reality, but never match it. It's a lab, for experimenting.
What MMA practitioners tend to say is that certain things will not work, now some are naive and say this because they didn't see anything like it on the UFC. Others say this because they are experienced and have tried this stuff, had it tried on them, been hit with it and come to realise, it's not reliable in a fight.
They will also offer up the proof, "try it yourself" and give the parameters to demonstrate the experiment, try it on a reasonably skilled fighter, at least in the skills the technique is supposed to work against.
Now of course there are situations where physical techniques can be used that are not fights, wrist locks, come alongs, off balancing, etc. Not to mention that a lot of stuff that doesn't work on skilled fighters, works really well on unskilled ones, better then the stuff that works on skilled ones even.
Another thing that I have found is that most MMA practitioners with reasonable experience are not prone to making claims about the "street", it is a uncontrolled environment with far too many variables. Anything can happen, and strange things can work.
What they usually advocate is that the best method to prepare, even though this should not be the goal, is having a solid base in all areas of fighting, and being able to adapt to different situations. And the best way to get this is by allowing yourself to be put in all sorts of situations through sparring with limited rules. Because if you end up in a fight, your best bet will be experience fighting live, even if the experience was gained in more controlled environments. Like if you suddenly have to fly a plane, you'd have been better logging hours in a simulator that gives many different sorts of problems to work around rather then just knowing the manual, which hasn't actually been tested in recent memory.
Where the feuds usually start is where a fanboy from either side comes in and starts making claims about what the other does. This can be a UFC fan claiming traditional styles are worthless because they don't work in the UFC. Or it can be from a traditional stylist claiming his fancy technique will KO anyone that trys to shoot on them.
One thing I have learnt is that it is near impossible to do everything. There simply isn't enough time. And the more you try to do, the less skill you will have at the individual parts. A MMA fighter is never going to be as good at boxing as someone that only boxes.
So we have different systems focusing on different areas. Some hit there area better then others, some don't seem to hit any area well...
But, the problems all seem to start when people make claims about there art fitting in other peoples areas of expertise. (ex. "No, you cannot drop to horse stance and drop and elbow to stop a shot, it's been tried, didn't work...")
Street fighting however, is outside everyones area, at least everyone not in jail anyways... so it gets all wierd.