Is it safe to use ceftin (cefuroxime) long term (1 year+) as an acne treatment?

Yes. If it's working, keep it up. Other antibiotics may work as well, or as you get older a topical agent such as Clindamycin may prove sufficient. I needed oral tetracycline for 25 years and am grateful for it.
Complicated. This oral cephalosporin has little if any direct side-effects other than occasional GI intolerance. Any broad-spectrum antibiotic can have secondary complications like c. Diff colitis, candidiasis, delayed hypersensitivity reactions, etc. The real question is whether the benefits outweigh the risks and only you and your doctor together can arrive at that decision.