gblues mentioned the scaling bug for DQ1/2 mobile, which sucks.
The only issues I can think of are the aspect ratio (they're portrait mode only) that led to weird scaling on the map, and the lack of buttons (can't say I minded).
I haven't seen how the versions unlocked in DQXI work, but I assume they fix this.
As far as Dragon Quest 3 goes, there's a few more issues then that. The battle animations from the SNES version are missing (despite using the same spritework), and the Pachisi minigame is also absent. The second bonus dungeon from the GBC version is also absent (which makes sense, as it's based off the SNES version)