You can have "safety issues" with any battery chemistry, and I've seen or heard of at least one example of every one of them failing in a dramatic fashion.
I don't use my SLA much anymore, except for experiments and to power stationary things (like a server-sized UPS whenever there are power outages).
Once I got NiMH (thanks to Ianmcnally), I used it in preference to the heavy SLA. I saved and scrimped and shorted myself on things and bought some Thundersky LiFePO4 cells, but didnt' quite have enough voltage to do what I needed with them, and was afraid to use them without a BMS after various problems I'd read on DIY Electric Car forums and elsewhere (like here), and was taking forever to build a BMS for them out of parts from a few people here on ES....
Later I got a damaged "prebuilt" LiFePO4 (thanks to Bluestreak), and after repairing it I used that in preference to the NiMH, because it was a little more power-dense, though not really any lighter than the NiMH, it had more usable power.
Later I started experimental LiPo-type packs, including good and repaired RC-LiPo from Mdd0127 and others including Ohzee, using this over the others because it is a lot more power-dense *and* it can supply a much higher current level than the others without as much voltage sag (although the SLA can supply jsut about any current levle I care to draw from it, it won't last very long at those rates and it sags so much that the higher current doesn't really result in that high of a total power level).
I havent' tried NiCd yet, htough I have a pile of old powertool packs to disassemble and rebuild, if I ever get the time to try it.
Same thing for laptop LiCo cells...got a lot of them but haven't gotten a pack built yet, despite starting several times.
Haven't tried A123 either, only have one cell of the larger cylindrical type, don't know it's condition.
So far I've found they all have their place, and their uses, but basically I prefer whatever has the best power and energy density, as it makes for the lightest smallest usable pack on my already pretty heavy bikes.
BTW: at one point I considered piling all of my batteries on a trailer to get me to Tucson for the Death Race, but it would be an all-day (12-18 hour) trip to get there at the state limit of 20MPH plus probable stops to fix things and rest and whatnot. Then I guesstimated the energy usage because of hills and slopes vs the weight of that trailer of batteries, and I would just about make it there then have to spend a week charging it all to get back.
That's assuming I didn't get run over by someone going 90MPH on the highway I'd have to be riding on....