When there is a lawsuit against you for unlawful training/exit fees, that would be the first warning. Locked in for 2 years, at £20,000 for training which comes nowhere near to that volume, and the fact they take 2/3rds if not more of the fee from the client to 'cover' this 20,000 - stay away.
Speaking of, pay is dreadful. Only in 2018 did they finally change their pay scale and introduce a london wage, but £23000/25000 for a 1st year BA, when Just Eat give £35000 for their grads.
In addition, 20 days holiday pa, lower than most companies. Low pension contributions, and no other benefits besides free mini tea/coffee in their academies.
If you're unlucky, you'll have a trainer who doesn't really care and sets the bare minimum work they can get away with. I was lucky and had multiple projects for different subjects, but I've spoken with people from other classes/locations whose trainer didnt care, set them no projects and didnt do more than read a pre-prepared powerpoint document which is already accessible to the students.
The BA training covered everything that a BA should do, not I did anything related on my client site; but for developers and testers I spoke with others who said their course was severely lacking in relevant and modern training.
No pay during training (can be up to 12 weeks + any time after when you haven't been placed) is embarrassing. Given how the expectation is that you can't have any job whilst working for FDM, I don't know how FDM expect their consultants to support themselves at all.
Once you're placed, don't expect to hear anything from your account manager. You're literally just another number to them and another commission check. You'll get the occasional visit if it fits in with their schedule when seeing the client, no emails either. Very surprising when you consider how 'chummy' they are with everyone back in the academies.
Most of HR are nice and are caring, except if it comes to leaving FDM/client early. No matter what you tell them as to reasons, even if they already knew about it, the idea you'll leave either one makes them flip 180.
This is from a UK perspective, I can't speak for those in America/Asia.