I bought a Lion Knight's Castle for my son a couple years ago. I built it myself to make sure all the parts where there, but I haven't given it to him yet. At the time, I decided he was too young, then we had another baby, starting the cycle all over again. We're moving to a bigger house this year, so I plan to give it to him then. But I regret buying it. It's a really cool castle, but here's the thing.
The big rock piece that came with it was snapped in half. So I emailed Playmobil, and they sent me another one, which was also broken, but it was still usable. This castle is totally useless without any one of those large pieces. When I bought it, I thought I could connect multiples together and re-arrange the different parts. Nope! You can only use it in the way it was designed. When a major part breaks, the rest of the set becomes totally useless.
I actually love System X. I haven't gotten any steck sets myself yet. But I have half a dozen of the Naval Stronghold sets, and from time to time, my son and I pull them out and make giant forts. It's a lot of fun. (And it takes up so much more space than I ever would have imagined!
) We also have a couple of the System X western forts.
I imagine the problem parents would have with System X is not so much how long it takes to assemble, but the time they have to spend RE assembling large sets, especially ones that involve large rock pieces, which don't hold the pins very well. But I don't think System WHY solves this problem either.