Like @Apomixis said, it depends on what you’re trying to achieve. Leraning to write characters by hand takes time, not just in the sense that you need to do it often and over time to remember, but also in the sense that writing a word takes much longer than reading it, so going through ten reading cards is many times faster than going through ten writing cards. Thus, you can’t really compare Skritter with apps that don’t teach you writing, because of course you’ll get further if you ignore part of the language. That’s not necessarily a bad thing, of course, I’m just pointing out that it’s not really comparable.
When it comes to the SRS, I think I adressed this in the other thread where you asked about the SRS (here in case anyone else wants to check). Like I said there, if you as a user decide (arbitrarily, for the sake of argument) how much to study per day, that means that under the hood, the app ignores the algorithm. This means that you still have a large number of items due, but it’s hidden from you as a user. Like I also said, we prefer not to hide this.
Obviously, I don’t have in-depth understanding of the algorithms and functionality of specific apps and don’t want to comment on any one in particular, but most SRS use roughly comparable algorithms, usually based on or derived from SuperMemo. The point is that they all work more or less the same. The difference here is probably not in the algorithm, but in what the app developers choose to show the users.