I've said this before, but on a more general basis, and leaving aside practicalities for a moment, I personally believe that stepmothers often have a far harder time of it - emotionally - than stepfathers anyway. There isn't quite the same mythology attached to men as there is to women ... e.g. the wicked stepmother and all that. As women, a large proportion of society still sees us as the nurturing sex, and we are therefore expected to fall far more easily into the role of caring for children who aren't our own than men are. This can mean we are immediately, and sometimes harshly, judged when we don't instantly fall in love with our stepchildren (even if we are kindness and understanding personified - sometimes in the most trying of circumstances). Should we dare express concerns, worries, or heaven forbid complaints I think we are jumped on (by some - both in real life and on boards like these) much sooner and much more than a man would be .... almost as if we're somehow being "unnatural" by failing to extend our motherly extincts, no matter what. By contrast, I think men tend to receive more understanding and sympathy ... I've heard, more than once, for example, a man being congratulated on being prepared to take on "another man's child" ... this is often seen as a hugely generous and charitable act, whereas a woman tends to be expected to take on her partner's children without any fanfare.
On the practical side .... while things are improving, I think it's still true to say, generally, that most households haven't yet embraced a completely fair division of labour - even when both parties work equal hours in paid employment - and women still tend to take responsibility for more than their fair share of housework, cooking and childcare. If you then bring extra bodies into the household on even a semi-regular basis, that inescapabily means her workload increases, because extra people mean more mess, more cleaning, more supervision. You don't have to be superbrain to see how women (usually) can therefore feel particularly resentful when "their" work increases as a result of their partners seeing their children .... obviously, in an ideal world, any extra work would be covered off by said partner without a murmur, but realistically, how often does that happen ? Women can also be faced with a hefty dose of emotional blackmail when it comes to their partners pulling their weight ... you get this whole "I don't want to spend precious contact time doing housework" argument which is all very well, but you know, normal life still goes on - even during contact - things still have to be cleaned, put away etc (even more so with extra kids about), so who then, exactly, is supposed to do this before the whole house descends into chaos ?
And all of the above applies to stepfamilies which are relatively sorted. However, if you have the misfortune to have a "difficult" (polite version) ex - which unfortunately, so very often also means "difficult" stepchildren, then damn right the stepmother usually gets a much harder time of it re: his kids compared to his experience of her (the stepmother's) children. If I added up all the hours we've spent going round and round in circles talking about various "ishoos" (ad bloody infinitum) relating to his kids, his ex, contact and so on, compared to the hours spent discussing my child - or indeed our child - I think you'd find the ratio is probably something like 10 to 1. It could even be higher. For the most part my/our kids just get on with life, but with his, it often feels like every last wretched thing is a great big drama, never simple, rarely normal. Quite apart from time, the emotional reserves invested in his kids has been much higher - and needless to say, a disproportionate amount of money has been/is being spent on his kids ( ... though that may surprise the step-bashers who'd like to believe that 2nd families thrive at the expense of the 1st .... errrr no, not always).
So yeah ...... sorry for the rant, but personally, I think it's been far far easier for DP to be a stepparent than it has been for me. My child hasn't caused him long term grief in the same way the many varied problems of my stepkids have caused me. Admittedly, a lot of this is due to his ex - though I firmly believe that divorced parents or not, all children should take at least some responsibility for their own behaviour as they get older (isn't that one of the main objectives of bringing kids up ?!) but the fact remains that regardless of "fault" it's a lot lot harder to deal with/have a relationship with his kids than it is for him to do the same with mine.