Congrats on the purchase- I'm late on chiming in but I have a little tidbits here and there.
Personally, I see no issue with keeping the 150 sanders flat, however I grew up using half sheet finish sanders for quite literally, EVERYTHING. I would edge sand cabinet frames, round over corners, flatten uneven table tops, sand through layers of paint- and basically, there was very little I couldn't do with it. I finally bit the bullet after wanting dust collection for so long and bought into Festool with an RO90 and the ETS EC 150/3 with the plan to (in the very near future now) purchase the RO150 which will compliment my ETS EC 150/3 nicely. The only sander I have issues keeping flat, and this is just small, usually it's been pretty good is the RO90.
The suggestions to always try beforehand if possible are great. I had no idea what a brute the RO90 was and while I'm midway between hobbyist and pro, but I use it a lot for apartment renos. For average all around, I think the finish sanders are they way to go, especially random orbits which are much better at taking down material than something like the 1/2 sheets.
A side note for power consumption we should all realize- latest apartment we had to put in some arc fault plugs and breakers- and the vac and ro90 work without tripping- however my Makita LS1016, one of their higher rated 10" sliding compound miter saws will trip the arc fault every time. I want to assume it has to do with the construction quality of the Festool, although it could just be the way the miter saw draws power to start up.