The maths of bonus conversions have affected two groups of players negatively of late. In the case of the UK, the bonus funds are kept separate and you play cash first which can be withdrawn at any time if you win with it, thus forfeiting the bonus. This means that for example on a 100% offer with £100 deposited, you only start chewing the wagering when the cash is lost, therefore only have £100 in bonus funds to play the wagering with. Before, when lumped together with the deposit, £200 all-bonus balance would have given you double the chance at level stakes of hitting a big win to help convert it (remember, in most cases even though the cash balance is kept separate, the wagering calculation is still the dep+bon amount.) So you have half the starting amount to attack this large wagering with.
In other countries that still have the lumping together of dep+bon into a single bonus funds amount with wagering being chewed right from the first spin, the wagering tends to be far higher than before with 25-40x dep+bon as the the target and massively odds-against in terms of conversion at even 96% average game RTP. As pointed out above, getting that is hard now with many 94% RTP sites which effectively reduce this already harsh wagering conversion chance by another 50%!! Bear in mind, 96 down to 94% has increased the house edge by half on top of the poor maths already in place from insane wagering multiples.
Roughly, a 30x bonus-only wagering at 96% on a £100 deposit with 100% bonus at £1 a spin would leave you with £80 from your starting £200 after meeting wagering so slightly EV- but with variance you would expect to convert the whole bonus £100 about 20-30% of the time. Reduce the RTP to 94% on the same deal and on average your £200 would turn into £20 after wagering, so the 50% increase in house edge would have an exponential effect on your chances, reducing them by a factor of 4x or in terms of your average end balance, by 75% so only on average around a 5-7% chance of conversion! Now you can see the futility of modern day bonus offers, why I haven't taken them for years.
Those examples above are for the common worldwide (and once the UK) system of tying in dep+bon into one bonus amount. The 30x bon example is quite generous nowadays. Imagine then a 25, 30 or 40x dep+bon wagering where that 5-7% chance on 94% slots would fall even further, to approximately 2.5-3.5% in the case of 30x dep+bon and a miserly 0.75-1.5% in the case of 40x dep+bon wagering. You've got next to no chance. Then use the UK example of only bonus funds counting towards wagering, and halve those figures again if you have a dep+bon requirement in your offer terms.
The huge majority of sign-up bonuses are almost pointless now, eye candy for the banners. The only attraction as a UK player is an odds-against 'parachute' chance whereby when you've spunked the cash part of your bankroll, there's a slim to anorexic chance you could still make money from the bonus wagering, even then in many cases the conversion or cash-out is restricted to 2,3,5 or 10x your deposit anyway.
The best long-term strategy if your casino has sign-up offers like the above and tends to give you repeat offers thereafter, is to choose the most volatile slot you can find and stick to it when you take bonuses.
