A while ago I bought some low-temperature lead-free solder paste CHIPQUIK SMDLTLFP.  
Today, I tried it in a mini toaster oven (without controller) and unfortunately I slightly over-cooked it on the first try because I had stuck a thermocouple to the PCB with aluminium tape and the tape’s adhesive failed at around 100 C.  With a suitable controller (i.e. one capable of learning a lower temp profile) I think this paste would be quite a safe option for sensitive boards.  
Lead-Free / RoHS Compliant
Description

Solder Paste Lead-Free Sn42/Bi57.6/Ag0.4 no clean Low Temperature Melts 138C 281F eutectic.

Alloy: Sn42/Bi57.6/Ag0.4
Flux Type: Synthetic No-Clean
Flux Classification: REL0
Metal Content: 87% metal by weight.
Particle Size: T3 (25-45 microns)
Melting Point: 138C (281F)
Size: 5cc/15g syringe
What are the negatives to using low-temp paste for DIY reflowed boards?