Weld-on is perfectly safe as it is used in every equipment within this hobby. No precaution beside the fact that it needs to dry/cure, which usually is around 24hrs. Weld-on is not like a glue that we think they are. It's more of a chemical solvent that when applied to acrylic, its will soften the acrylic to a "soft" form and will bond the 2 piece together hence the word "weld "- On.
Silicone is only safe if the correct one is used. Silicone and acrylic are not compatible , as acrylic chemical base is too smooth for silicone to bond. For repair or minor DIY is perfectly fine. But i will not use silicone to bond acrylic if, it's use outside of the sump or DT or any thing pressurized.
Imho, I think every hobbyist should have a tube of weld-on in there fish room, hoarding room, garage, or the ex-wife's room.. Haha