Support depends where you buy the license from. If you buy direct you get the best Blesta support you can ever want.
If you buy from a reseller (Distributor / Reseller webhost [eg: Site5,etc]) you get support from them, and that depends per business you use. You get support here as-well as the others above.
As for stable, Blesta is the most stable billing system out there on the market. If you look at the bug fixes for Blesta and then a competitor and you will see the difference. Blesta 1 - 5 in a beta, maybe a few got outside beta maximum, WHM** - ClientExec you get about 100 and / or more per release. And most of them are for simple things they just didn't bother with.
As for Security, Blesta again is the most secure billing system out there, and don't just take my word for it read these:
- http://www.webhostingtalk.com/showpost.php?p=8898652&postcount=14
- http://www.webhostingtalk.com/showpost.php?p=8885045&postcount=183
Two well known security auditors.