Cordless nailers are good but it depends whether they are good for you or not. I assume you're on about the 16g and 18g nailers and not the big Paslode style guns.
Cordless nailers are definitely handier for smaller workshops that don't have space for a compressor or worksites where a compressor and hose is just an extra hassle. But the tool companies know that and they price the tools accordingly which means they aren't cheap. The only real "Inexpensive" Cordless nailers worth a look are the Ryobi ones and they're still in the £100 - £150 range without a battery or charger. I've seen a few cordless nailers on worksites and I've seen them break down as well. All the while the air nailer keeps on going without blinking an eye, And I've never seen one break, Only wear out after years of heavy use.
It really does depend on circumstance you're in. Personally, If I'm not doing it every day I would just use a hammer and tacks. I used to put together boxed sashes like that before I bought a 16G air nailer for the job. Can't get any cheaper than that.