diff --git a/hw/audio/pcspk.c b/hw/audio/pcspk.c
index 8e3e178..f980d66 100644
--- a/hw/audio/pcspk.c
+++ b/hw/audio/pcspk.c
@@ -192,8 +192,9 @@ static void pcspk_class_initfn(ObjectClass *klass, void *data)
 
     dc->realize = pcspk_realizefn;
     set_bit(DEVICE_CATEGORY_SOUND, dc->categories);
-    dc->cannot_instantiate_with_device_add_yet = true; /* FIXME explain why */
     dc->props = pcspk_properties;
+    /* Reason: pointer property "pit", realize sets global pcspk_state */
+    dc->cannot_instantiate_with_device_add_yet = true;
 }
 
 static const TypeInfo pcspk_info = {
diff --git a/hw/block/fdc.c b/hw/block/fdc.c
index 86f4920..592b58f 100644
--- a/hw/block/fdc.c
+++ b/hw/block/fdc.c
@@ -2234,7 +2234,6 @@ static void isabus_fdc_class_init(ObjectClass *klass, void *data)
 
     dc->realize = isabus_fdc_realize;
     dc->fw_name = "fdc";
-    dc->cannot_instantiate_with_device_add_yet = true; /* FIXME explain why */
     dc->reset = fdctrl_external_reset_isa;
     dc->vmsd = &vmstate_isa_fdc;
     dc->props = isa_fdc_properties;
diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index fe33843..20402ba 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -544,10 +544,15 @@ static void port92_class_initfn(ObjectClass *klass, void *data)
 {
     DeviceClass *dc = DEVICE_CLASS(klass);
 
-    dc->cannot_instantiate_with_device_add_yet = true; /* FIXME explain why */
     dc->realize = port92_realizefn;
     dc->reset = port92_reset;
     dc->vmsd = &vmstate_port92_isa;
+    /*
+     * Reason: unlike ordinary ISA devices, this one needs additional
+     * wiring: its A20 output line needs to be wired up by
+     * port92_init().
+     */
+    dc->cannot_instantiate_with_device_add_yet = true;
 }
 
 static const TypeInfo port92_info = {
diff --git a/hw/input/pckbd.c b/hw/input/pckbd.c
index dee31a6..655b8c5 100644
--- a/hw/input/pckbd.c
+++ b/hw/input/pckbd.c
@@ -522,7 +522,6 @@ static void i8042_class_initfn(ObjectClass *klass, void *data)
     DeviceClass *dc = DEVICE_CLASS(klass);
 
     dc->realize = i8042_realizefn;
-    dc->cannot_instantiate_with_device_add_yet = true; /* FIXME explain why */
     dc->vmsd = &vmstate_kbd_isa;
 }
 
diff --git a/hw/input/vmmouse.c b/hw/input/vmmouse.c
index 600e4a2..6a50533 100644
--- a/hw/input/vmmouse.c
+++ b/hw/input/vmmouse.c
@@ -282,10 +282,11 @@ static void vmmouse_class_initfn(ObjectClass *klass, void *data)
     DeviceClass *dc = DEVICE_CLASS(klass);
 
     dc->realize = vmmouse_realizefn;
-    dc->cannot_instantiate_with_device_add_yet = true; /* FIXME explain why */
     dc->reset = vmmouse_reset;
     dc->vmsd = &vmstate_vmmouse;
     dc->props = vmmouse_properties;
+    /* Reason: pointer property "ps2_mouse" */
+    dc->cannot_instantiate_with_device_add_yet = true;
 }
 
 static const TypeInfo vmmouse_info = {
diff --git a/hw/intc/i8259_common.c b/hw/intc/i8259_common.c
index 2acdbfe..9d29399 100644
--- a/hw/intc/i8259_common.c
+++ b/hw/intc/i8259_common.c
@@ -135,9 +135,15 @@ static void pic_common_class_init(ObjectClass *klass, void *data)
     DeviceClass *dc = DEVICE_CLASS(klass);
 
     dc->vmsd = &vmstate_pic_common;
-    dc->cannot_instantiate_with_device_add_yet = true; /* FIXME explain why */
     dc->props = pic_properties_common;
     dc->realize = pic_common_realize;
+    /*
+     * Reason: unlike ordinary ISA devices, the PICs need additional
+     * wiring: its IRQ input lines are set up by board code, and the
+     * wiring of the slave to the master is hard-coded in device model
+     * code.
+     */
+    dc->cannot_instantiate_with_device_add_yet = true;
 }
 
 static const TypeInfo pic_common_type = {
diff --git a/hw/misc/vmport.c b/hw/misc/vmport.c
index 94ae6ae..cd5716a 100644
--- a/hw/misc/vmport.c
+++ b/hw/misc/vmport.c
@@ -162,7 +162,8 @@ static void vmport_class_initfn(ObjectClass *klass, void *data)
     DeviceClass *dc = DEVICE_CLASS(klass);
 
     dc->realize = vmport_realizefn;
-    dc->cannot_instantiate_with_device_add_yet = true; /* FIXME explain why */
+    /* Reason: realize sets global port_state */
+    dc->cannot_instantiate_with_device_add_yet = true;
 }
 
 static const TypeInfo vmport_info = {
diff --git a/hw/timer/i8254_common.c b/hw/timer/i8254_common.c
index dc2196c..9db5c9d 100644
--- a/hw/timer/i8254_common.c
+++ b/hw/timer/i8254_common.c
@@ -282,7 +282,12 @@ static void pit_common_class_init(ObjectClass *klass, void *data)
 
     dc->realize = pit_common_realize;
     dc->vmsd = &vmstate_pit_common;
-    dc->cannot_instantiate_with_device_add_yet = true; /* FIXME explain why */
+    /*
+     * Reason: unlike ordinary ISA devices, the PIT may need to be
+     * wired to the HPET, and because of that, some wiring is always
+     * done by board code.
+     */
+    dc->cannot_instantiate_with_device_add_yet = true;
 }
 
 static const TypeInfo pit_common_type = {
diff --git a/hw/timer/m48t59.c b/hw/timer/m48t59.c
index f81cf48..8005503 100644
--- a/hw/timer/m48t59.c
+++ b/hw/timer/m48t59.c
@@ -750,9 +750,10 @@ static void m48t59_isa_class_init(ObjectClass *klass, void *data)
     DeviceClass *dc = DEVICE_CLASS(klass);
 
     dc->realize = m48t59_isa_realize;
-    dc->cannot_instantiate_with_device_add_yet = true; /* FIXME explain why */
     dc->reset = m48t59_reset_isa;
     dc->props = m48t59_isa_properties;
+    /* Reason: needs to be wired up by m48t59_init_isa() */
+    dc->cannot_instantiate_with_device_add_yet = true;
 }
 
 static const TypeInfo m48t59_isa_info = {
diff --git a/hw/timer/mc146818rtc.c b/hw/timer/mc146818rtc.c
index 2f58220..17e1907 100644
--- a/hw/timer/mc146818rtc.c
+++ b/hw/timer/mc146818rtc.c
@@ -906,9 +906,10 @@ static void rtc_class_initfn(ObjectClass *klass, void *data)
     DeviceClass *dc = DEVICE_CLASS(klass);
 
     dc->realize = rtc_realizefn;
-    dc->cannot_instantiate_with_device_add_yet = true; /* FIXME explain why */
     dc->vmsd = &vmstate_rtc;
     dc->props = mc146818rtc_properties;
+    /* Reason: needs to be wired up by rtc_init() */
+    dc->cannot_instantiate_with_device_add_yet = true;
 }
 
 static const TypeInfo mc146818rtc_info = {
