I'm a big fan of balance bikes too. They all work just fine, but you mostly get what you pay for. I've seen Islabike Rothans ridden by large grown men, I'm not sure I'd trust all the balance bikes I've seen to withstand that. But then all you have to do is stop the grown men riding them and the problem is solved 😉
As long as they fit well then they'll be fine. For a good stride the saddle needs to be quite low, which is why the option of taking the pedals off a little bike often isn't very good. So check inside leg measurements against specs (or the real thing if possible). Cranks get in the way too a bit. The simplicity is a big thing, nothing in the way and nothing to distract – I'm still not sure if brakes are good or not on balance bikes.
The best thing about balance bikes is that they're not just a stepping stone to pedalled bikes – they're just brilliant fun. They can explore bike handling in a different way than with a pedalled bike without the additional stress/complication.
Our oldest is 5 1/2 and still has fun messing about on his Rothan*, and his Cnoc 14, as well as riding his Beinn 20. Call me an Islabike fanboy if you like but in the end nothng quite matched up on spec and weight 🙂
* will be his brother's next summer sometime I think!