Yes Very often it happens that a weak team suddenly plays in the UCL league and when they play in the UCL there is no significant resistance, instead it becomes a source of goals for strong teams and the most fatal thing is that they become less consistent in the league and even their performance decreases. Like the example you gave of the Lens team, this could also happen with Brest if they don't buy players and strengthen the squad for next season because of course with Brest's entry into the UCL league their schedule will become even busier so it will affect the players' fitness which makes them unable to maintain their performance.
I admit that Brest has really improved this season. They are quite big now and in the near future, they will be playing among the big players in Europe. Next season's UCL League will not only mark the start of a new era, as we see 36 teams in the league stage, rather than the group stage with 32 teams. We'll also see the arrival of new arrivals, notably Brest, Girona and Bologna, as well as the return of 1982 champions Aston Villa and Stuttgart, whose last appearance in the Champions League was in 2010.
Of course the smaller teams when they have presence in Europe, it’s most time challenging for them as they ought to go through that and also their domestic league. Going through these might be one hectic one amd as such, you see them fumbling in the group stage of the competition. Well there’s sure a way to Avoid this kind of situation and I’ll take a run down.
One of the best way is to build squad depth. Doing this will help the club and coach be able to do needed rotations as such resting the players in most matches and not overworking them beyond their normal stretch. I think participating in European competition also brings in more finance, so it will be best to be able to balance the budget, so as not to overspend and get caught with FFP or short spend and lose out on all competitions.
I really want the small teams coming to Europe to make a name and there’s no way than them building a strong squad and maintaining consistency especially in domestic performance. Yes, it’s a great thing to qualify for the UCL but with it’s new format of 36 teams, you don’t know what to expect so as such the smaller teams shouldn’t fail on club and squad building.