In every business there is a balance between the need to keep cashflow rolling and really difficult and unprofitable work.
In professional services Firm's such risks are higher given the high human capital cost with limited ability to leverage man hours.
At LCC we have a strict approach of looking at potential engagements and attempting to avoid what I have labelled 'vortex's'. These are situations that might have cashflow attached but are very difficult pieces of work to complete.
They occur in every industry. Whilst it is always a balancing act, I recommend that you critically review larger opportunities that are presented where the profit is marginal. If such an opportunity / contract proved to be heavily unprofitable the associated 'stress' that it can place on the corporate system can be incredibly negative.
In my experience it is far better to avoid these situations and pursue other, more profitable, work.