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Interesting and thorough research that you've done here. (We expect nothing less from yourself).
So, in summary:
Spreadex are no saints, Plus500 are not either - both appear to attempt to (and appear to be successful at) influencing on-line reviews for their spreadbetting sites.
This is far from desirable but certainly not uncommon on many on-line review sites. I always wonder if the hotel reviews are written by the owners mate. You have to discount some of the reviews.
The AIM Prospector Editor may be completely oblivious to the lack of independence whilst being sponsored by Spreadex and writing negative reviews about their competitors. Maybe not.
Regardless, some interesting points are raised in the AIM Prospector article and you have put some interesting points that tar Spreadex with the same brush - but all it does is suggest that both parties employ some dubious methods. However, it is not uncommon for business competitors to employ underhand tactics in an attempt to gain competitive advantage.
You point out that both Spreadex and Plus500 are posting very good figures with similar EBITDA, both appear to be thriving growth businesses.
What we can't say is if these figures have been materially assisted by dishonest marketing.
Detractors say that the source of revenue for these type of companies is from 'mug punters' and that it could dry up - 'mug punters' have been around for a very long time betting on horses, boxing, football, cricket, greyhounds, and anything else that takes their fancy - I don't see any sudden wising up of the human race in the near future. Maybe 'mug punters' are easily swayed by on-line reviews too - would make it quite tempting to improve your ratings by fair means or foul. Their battle for market share will continue. In the meantime the figures look good.