That's definitely not true. I joined Symbian in 2005 and there were lots of licensees around. I think 2005/2006 were the glory years of the company.
Agree with the ugly. Well, beauty is in the eye of the beholder. Some say it was an engineering master piece of the 1990s. But it was different from everything else and it was difficult, I don't think this can be disputed.
The no good reason is not the whole truth:
* When Symbian (then called EPOC32) was developed there was no C++ compiler that supported exceptions without major problems. "Don't use them" was typical in style guides at the time. So EPOC came up with their own macro hackeries instead, which worked when used correctly. The latter was too difficult for many developers.
* Early devices had just single digits of megabytes of RAM. So 500 times less than a phone of today. More macro trickeries to save Bytes were the answer.
Around 2003:ish Nokia was the only Symbian licensee left and they had even 3 UIs: Series 60, Series 80 and Series 90. Only Series 60 survived. Series 80 had 2 products and Series 90 one public prototype. Series 90 was the only with touch input originally. Series 60 got touch after the iPhone, but that was too late.
Yes, Symbian C++ was too difficult even inside Nokia. Productivity was very low, many products got cancelled before seeing the market and those who did were massively delayed.