Why the PS3 WAS hard to develop for (and why this is no longer the case)

The PS3 is still hard to develop for. What's changed is that they've finally shipped enough units to make ports worthwhile.

The 360 has three symmetric cores and a decent GPU. The PS3 has a single core, a comparable GPU - and six very constrained SPUs that you'd better learn to juggle if you want your PS3 game to even approximate the quality of a 360 title. There are more than a few games that crippled their 360 version to match what they could achieve on the PS3.

To be fair, the PS3 is easier to develop for than the PS2 and it has gotten easier over time, but if you take the 360 as the benchmark the PS3 will always be hard in comparison.

>in fact some people claimed that the PS3 had nearly “unlimited power”

These people are known as "marketers".