Former professional poker player here, no limit holdem cash games, mid stakes. NLHE cash games are considered the hardest to solve given the depth of the decision trees. Other formats like sit and gos, spin and gos, and tournaments can be modeled and solved in a more simplistic way.
A group of us (cash game pros) discovered a group of players on the Ongame network around 2010. We combined our databases and found a few dozen players that all shared an unconventional style and had outstanding winrates.
They made very annoying, aggressive plays that were difficult to counter, not taught anywhere in professional videos, forums or training sites, and we're winning at a world class winrate.
All the regular pros talk and knew each other, and no-one knew who these players were.
Combined they took millions in winnings out of the network. We presented our collaborative research and investigation to Ongame and they didn't do a thing about it.
In more recent years players on Pokerstars have received refunds when their security teams have deemed them to have lost to bots.
Pokersnowie is the most advanced commercially available bot and they even have an API now.
I quit playing in 2011 and would strongly advise people staying away from online poker. It's a scummy world, the games are tougher than ever, the sites are shadier in their operation, run higher margins, and with the current progress in the AI/ML domains you're on a hiding to nowhere.
I was skeptical but saw this evidence first hand and lost thousands personally, and this was 5 years ago. Anyone playing online in 2016 that doesn't believe they're up against super sophisticated bots are deluded.
Happy to share more if people are interested.