I feel like there is an element of truth in being concerned about taking supplements. There are SO many different kinds and so many different companies and not a lot of regulation about them. I think there are certainly brands that I wouldn't have a problem taking, but I do think that those choices should be well researched. I think many practitioners have a general perspective about the pros and cons of supplements, and depending on their experiences with them will tend to be all or nothing about them (along with other treatments based on cases they tend to see regularly or have a particular interest in). Supplements can be a sticky subject just based on the fact that they are not regulated very well, and it's definitely possible to make yourself sick with certain combinations of things, or too much of things, or things may be contaminated, and since a lot of supplements are "self prescribed", it can be hard to zero in on things that are or could be causing problems.
Personally the only supplements I take are Fermented Cod Liver Oil and Probiotics. I don't take the FCLO every day, only when I have a hankering for it, and it typically tastes "good", but every so often it starts to taste "bad" to me, and I take that as a sign that my body doesn't need it, so I lay off for a while. I take the probiotics to supplement the fermented foods I eat, just so that I have a "back up" if I'm not eating as many ferments as I would like. I am trying to make sure that my intake of iron, calcium, magnesium, vitamin K, vit D, vit A, and fat, amongst others are up there, and that I am not eating things or cooking things in a way that would impede absorption of those things.
When DS was a baby he had a lot of food intolerances and it made me do a lot of research into food and diet, and making changes in my diet allowed me to uncover that I had a lot of issues as well. I had to do food diaries and elimination diets in order to help out DS, and I got used to really listening to what my body needed and what made it feel "right" and what made me feel "off". Most supplements have binders and fillers and make me feel "off" (as well as bothering DS). Or it just might be that the proportion of vitamins/minerals just wasn't "right" for me (i.e. there are certain ratios and combinations of minerals that are ideal for functioning and an imbalance in these can actually cause psychotic symptoms), so I decided that I would just eat real healthy foods, and I have been doing really well ever since. Plus many vitamins are synthetic or are in a form that your body needs to convert to use, and in order to convert them you need specific precursors, which would typically come from eating a well balanced diet anyway...and many of the vitamins are fat soluble, so you would need a nice hefty dose of fat to absorb many of those vitamins, so they are mega dosed in the hopes that more will be absorbed.
I think that as long as you are looking into the companies that you are getting the vitamins from (particularly the fish oils if you are worried about heavy metal or chemical contamination...and the source of the fish makes a difference too), and making sure that you aren't megadosing on things that should be more balanced, you should be fine. But it is something that I would be careful about doing. HTH!