Yes, a lot of fx with stereo in/out sum to mono. But claiming all except compressors (and variants of it) do is absurd! Especially since a lot of (not dual mono) compressors do that as well with their internal sidechain path - which is the reason why results can vary when using a dual mono compressor linked or unlinked (with all controls matched, of course).
How about stereo EQs? True stereo reverbs like Bricasti? All the stereo algorithms on a H3000? All actual stereo delays (e.g., Symetrix 606)? Stereo distortion units (e.g., Elysia Karacter)?
I agree that stereo sends are not really necessary 99% of the time; luckily so, since - as mentioned - it is hard to source them (outside of DJ mixers). But that doesn't mean there are no true stereo fx besides compressors.
I am pretty sure that the thread started did not imagine this kind of equipment, especially if mentioning that the budget for a mixing console is below 700GBP.
Bricasti, oh yeah this kind of high end... this will certainly light up G.A.S. for me, even if not having a pop star like applications. Still so far what I have done is having just Lexicon at my horizont, and so far I use BEHRINGER V-VERBs (the design is good, and the manufacturing flaws with ground loops and psu noise I have ironed out with my modifications).
As for the ADT summing mixer, I just mentioned that since it was one of the few things that came to my mind with tons of stereo inputs (but still pan and level controls). Yes, it is expensive. But they are incredible pieces of gear. YMMV.
I really dont know why expensive? Becouse people are lazy to build it or they do not know that it is
incredibly primitive piece of gear, like a box with a volume control knob in it?
(and now after looking 3000GBP expensive effect units, lets imagine what would think a man using just VSTs (someone outside VSE forum)...